dev-perl/Test-Deep: Weaken dependency on dev-perl/Test-Tester

dev-perl/Test-Deep: Weaken dependency on dev-perl/Test-Tester

Without this fix, Test-Deep effectively needs perl-5.22
stabilized in order to satisfy Test-Tester from Perl-5.22

However, satisfying Test-Tester from a mechanism other than Perl-5.22
confuses portages dependency resolver, and confuses portage to not be able
to upgrade perl, and not being able to upgrade perl means perl can't be
stabilized, invoking a stabilization-needs-stabilization bug.

Hence, the choice here optimises for assuming stabilization will
happen/has happened, and removes any dependency on Test::Tester, assuming
portage will eventually provide it for you.

And just to be sure, we use perl before the test phase executes
to determine if Test::Tester was /actually/ installed and then only run tests
if it was.

This means before stabilization, some people *might* install this and not have
tests run, but everyone *will* get tests working after stabilization, as
opposed to the opposite occurring and stabilization being blocked.

Bug: https://bugs.gentoo.org/584238

Package-Manager: portage-2.3.0
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