Some testlib selftests don't behave well when logging test output to a file

The following tests either run multiple test objects or run the same test object multiple times. In both cases, this causes the output file to be overwritten for each run, so that the output for all but the last run is overwritten. These tests are therefore currently excluded from tst_selftest when logging to files.

benchliboptions – runs two test objects
multiexec – runs the same test five times
qexecstringlist – ignores the command-line in five out of six tests witht he same test object

Additionally, the differentexec selftest ignores the command-line and makes its own argv, and thus also fails when tst_selftest tries to log the test output to a file.

These tests must either be fixed to play nicely with tst_selftest or be removed if it is determined that they are exercising invalid use-cases for testlib.