To ensure Perl invocations within tests call the same Perl binary used
for running the tests, as specified or auto-detected via `PERL` env
(autotools) or `PERL_EXECUTABLE` setting (cmake). Instead of the first
`perl` executable found in `PATH`.)
Cherry-picked from #14949Closes#15097
Also introduce 'notexists' for verifying that directory entries do not
exist after a test. Now an explicit supported feature instead of using
"funny" perl in postcheck.
Closes#15046
Python precheck/postcheck alternatives were included but commented out.
Since these are not used and perl is guaranteed to be available to run
the perl versions anyway, the Python ones are removed.