Do not provide empty do-test target for perl5-using ports

perl5's do-test: target should only be enabled when USE_PERL5=configure
or USE_PERL5=modbuild* are used, otherwise it'll end up being empty
and will prevent other tests from running.

Remove NO_PERL5_TEST as it isn't really needed after this fix.

Approved by: mat, sunpoet
Differential Revision: D3830