Introduce a TAP_TESTS_PERL primitive to list test programs written in perl.
Only do this in tap.test.mk because I only expect perl-based test programs
with this interface.
This is very similar to TAP_TESTS_SH but the difference is that we record
in the Kyuafile that the test program requires a perl interpreter. This
in turn makes Kyua mark the test as skipped if the perl package is not yet
installed, instead of mysteriously failing to run the program.
MFC after: 5 days
e8de5c5235