Support perl-based TAP-compliant test programs.

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

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