meson: stop requiring Perl when tests are enabled

meson: stop requiring Perl when tests are enabled

The Perl interpreter used to be a strict dependency for running our test
suite. This requirement is explicit in the Meson build system, where we
require Perl to be present unless tests have been disabled.

With the preceding commits we have loosened this restriction so that it
is now possible to run tests when Perl is unavailable. Loosen the above
requirement accordingly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>