tests: add and use %PERL variable to refer to the Perl binary

[vszakats]

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 #14949

[dfandrich]

Analysis of PR #15097 at 9c4dd876:

Test 313 failed, which has NOT been flaky recently, so there could be a real issue in the PR.

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