t: adapt existing PERL prerequisites

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>