lib/googletest: Move to a new googletest package

gtest/gmock are not normally used by users unless running the tests,
so they shouldn't be in the utilities package. Move them to a new
googletest package, to match what we did with ATF/Kyua.

While here, move tests dependencies from tests-all.ucl to tests.ucl,
which is the canonical place for that.

This change adds a new package to the system so, until we have a proper
policy on how to handle this in release/stable branches, it should not
be MFC'd.

MFC after: never
Reported by: emaste
Reviewed by: manu
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53607