t: adapt existing PERL prerequisites
A couple of our tests depend on the PERL prerequisite even though it isn't needed. These tests fall into one of the following classes: - The underlying logic used to be implemented in Perl but isn't anymore. Here we can simply drop the dependency altogether. - The test logic used to depend on Perl but doesn't anymore. Again, we can simply drop the dependency. - The test logic still relies on a Perl interpreter. These tests should use the newly introduced PERL_TEST_HELPERS prerequisite. Adapt test cases accordingly. Note that in t1006 we have to introduce another new prerequisite depending on whether or not the IPC::Open2 module is available. Funny enough, when starting to use `test_lazy_prereq` to do so we also get a conflict of variables with the "script" variable that contains the Perl logic because `test_run_lazy_prereq_` also sets that variable. We thus rename the variable in t1006 to "perl_script". Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>