googletest: backport GTEST_SKIP to googletest 1.8.1

This commit backports revisions 00938b2b228f3b70d3d9e51f29a1505bdad43f1e and
59f90a338bce2376b540ee239cf4e269bf6d68ad from googletest's master branch to
our included version of googletest, which is based on 1.8.1. It adds the
GTEST_SKIP feature, which is very useful for a project like FreeBSD where
some tests depend on particular system configurations.

Reviewed by: ngie
Obtained from: github.com/google/googletest
MFC after: 2 months
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/rS345331