Add the save_test_env and reset_test_env procedure to record original
test environment (global and environment variables) and restore it once
a test set has been done.
Remove the --base64-failed-log option from the mt script as there is no
more need to output full test output serialized since the CI systems now
used can upload the test log files as artifact.
Abort download of icdiff, nagelfar or tclsh83 after 2 timeouts of 5
seconds and record download failure for icdiff to avoid mt script to
attempt a new download tentative every time it is called.