lint: fix SC2004 shellcheck warning

testsuite/bin/install_test_sh

@@ -102,7 +102,7 @@ case "$mode" in
       for cmd in ${cmdlist}; do
          unset IFS
          eval "$cmd"
-         ret=$(($ret+$?))
+         ret=$((ret+$?))
          IFS=';'
       done
       unset IFS

testsuite/lint.00-init/005-init_ts.exp

@@ -108,7 +108,7 @@ array set file_lint_opts [list\
    init/sh {-e SC2089,SC2090}\
    init/bash_completion {-e SC2207}\
    script/add.modules {-e SC2162}\
-   testsuite/bin/install_test_sh {-e SC1090,SC2004,SC3037,SC3044}\
+   testsuite/bin/install_test_sh {-e SC1090,SC3037,SC3044}\
 ]
 
 proc set_linter_opts {fpath} {