Initial googlemock/googletest integration into the build/FreeBSD test suite

Initial googlemock/googletest integration into the build/FreeBSD test suite

This initial integration takes googlemock/googletest release 1.8.1, integrates
the library, tests, and sample unit tests into the build.

googlemock/googletest's inclusion is optionally available via `MK_GOOGLETEST`.
`MK_GOOGLETEST` is dependent on `MK_TESTS` and is enabled by default when
built with a C++11 capable toolchain.

Google tests can be specified via the `GTESTS` variable, which, in comparison
with the other test drivers, is more simplified/streamlined, as Googletest only
supports C++ tests; not raw C or shell tests (C tests can be written in C++
using the standard embedding methods).

No dependent libraries are assumed for the tests. One must specify `gmock`,
`gmock_main`, `gtest`, or `gtest_main`, via `LIBADD` for the program.

More information about googlemock and googletest can be found on the
Googletest [project page](https://github.com/google/googletest), and the
[GoogleMock](https://github.com/google/googletest/blob/v1.8.x/googlemock/docs/Documentation.md)
and
[GoogleTest](https://github.com/google/googletest/tree/v1.8.x/googletest/docs)
docs.

These tests are originally integrated into the build as plain driver tests, but
will be natively integrated into Kyua in a later version.

Known issues/Errata:
* [WhenDynamicCastToTest.AmbiguousCast fails on FreeBSD](https://github.com/google/googletest/issues/2172)

Reviewed by:	asomers
Approved by:	emaste (mentor)
MFC after:	2 months
Differential Revision: https://reviews.freebsd.org/D19551