Import GoogleTest 1.8.1 into the vendor tree under ^/google/googletest/dist

GoogleTest is a widely used opensource C++ test framework, licensed under a
BSD 3-clause license. It fits best in the realm of doing functional/whitebox
testing, similar to ATF's C++ library. However, it has additional functionality
such as per-testcase setup fixtures, class level setup and teardown fixtures,
and a lot more functional/syntactic goodness.

In addition to large corporations adopting GoogleTest as their defacto C++ test
library (Facebook, Google, etc), many opensource projects have adopted
GoogleTest, e.g., the Capsicum project, Chrome, etc.

The goal for importing this is to enable testing with zfsd and integrate
googletest into kyua.

This is the final version that will support a pre-C++-11 compiler. As such, this
test framework will not be available to gcc 4.2.1, similar to ATF's C++ library.

A subsequent set of commits will:

1. Tag ^/google/googletest/dist as ^/google/googletest/1.8.1 using svn cp ^/google/googletest/dist ^/google/googletest/1.8.1.
2. Import this code into ^/head and integrate it into the build for all applicable platforms and C++ compile toolchains.

The import was done via the following command pipeline on OSX:

curl -L https://github.com/google/googletest/archive/release-1.8.1.tar.gz | tar --strip-components=1 -xvzf - -C dist/

Approved by: emaste (mentor)
Discussed with: brooks, jtl
Differential Revision: https://reviews.freebsd.org/D18622