Let's make a difference between places where Perl is required for the
test (AT_PERL_REQUIRE), and the places where it's used to run the
test, but it's not not to run the test (AT_PERL_CHECK).
* tests/local.at (AT_REQUIRE): New.
(AT_PERL_CHECK, AT_PERL_REQUIRE): New.
Use them where appropriate.
* tests/local.mk ($(TESTSUITE)): Beware not to start the line with
'-pi' if Perl is empty, as Make understands this as "it's ok to fail".
Which it is not.
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