ts: re-license test libraries from GPLv3+ to GPLv2+

Change license of all testutil-* libraries, used to test Modules Tcl
extension library, from GPLv3+ to GPLv2+.

Align this way all files from the Modules project under the GPLv2+
license.

Consent has been obtained from copyright holder to perform such move (I
hold the full copyright for these librairies).

Related to #389

lib/testutil-0getgroups.c

@@ -1,11 +1,11 @@
 /*************************************************************************
  *
  * TESTUTIL-GETGROUPS.C, Superseded getgroups function for test purpose
- * Copyright (C) 2020 Xavier Delaruelle
+ * Copyright (C) 2020-2021 Xavier Delaruelle
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
+ * the Free Software Foundation, either version 2 of the License, or
  * (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,

lib/testutil-closedir.c

@@ -1,11 +1,11 @@
 /*************************************************************************
  *
  * TESTUTIL-CLOSEDIR.C, Superseded closedir function for test purpose
- * Copyright (C) 2019 Xavier Delaruelle
+ * Copyright (C) 2019-2021 Xavier Delaruelle
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
+ * the Free Software Foundation, either version 2 of the License, or
  * (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,

lib/testutil-dupgetgroups.c

@@ -1,11 +1,11 @@
 /*************************************************************************
  *
  * TESTUTIL-DUPGETGROUPS.C, Superseded getgroups function for test purpose
- * Copyright (C) 2020 Xavier Delaruelle
+ * Copyright (C) 2020-2021 Xavier Delaruelle
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
+ * the Free Software Foundation, either version 2 of the License, or
  * (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,

lib/testutil-getgrgid.c

@@ -1,11 +1,11 @@
 /*************************************************************************
  *
  * TESTUTIL-GETGRGID.C, Superseded getgrgid function for test purpose
- * Copyright (C) 2020 Xavier Delaruelle
+ * Copyright (C) 2020-2021 Xavier Delaruelle
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
+ * the Free Software Foundation, either version 2 of the License, or
  * (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,

lib/testutil-getgroups.c

@@ -1,11 +1,11 @@
 /*************************************************************************
  *
  * TESTUTIL-GETGROUPS.C, Superseded getgroups function for test purpose
- * Copyright (C) 2020 Xavier Delaruelle
+ * Copyright (C) 2020-2021 Xavier Delaruelle
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
+ * the Free Software Foundation, either version 2 of the License, or
  * (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,

lib/testutil-getpwuid.c

@@ -1,11 +1,11 @@
 /*************************************************************************
  *
  * TESTUTIL-GETPWUID.C, Superseded getpwuid function for test purpose
- * Copyright (C) 2020 Xavier Delaruelle
+ * Copyright (C) 2020-2021 Xavier Delaruelle
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
+ * the Free Software Foundation, either version 2 of the License, or
  * (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,

lib/testutil-mktime.c

@@ -1,11 +1,11 @@
 /*************************************************************************
  *
  * TESTUTIL-MKTIME.C, Superseded mktime function for test purpose
- * Copyright (C) 2020 Xavier Delaruelle
+ * Copyright (C) 2020-2021 Xavier Delaruelle
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
+ * the Free Software Foundation, either version 2 of the License, or
  * (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,

lib/testutil-time.c

@@ -1,11 +1,11 @@
 /*************************************************************************
  *
  * TESTUTIL-TIME.C, Superseded time function for test purpose
- * Copyright (C) 2020 Xavier Delaruelle
+ * Copyright (C) 2020-2021 Xavier Delaruelle
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
+ * the Free Software Foundation, either version 2 of the License, or
  * (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,