Use static and global environment setup/teardown in C++ unit tests

[wmiaw]

Google Test supports static and global environment setup/teardown which will allow expensive initialization like the MockMslContext and other constants to be initialized one during unit tests instead of on every single test like it is currently implemented. This should help speed up execution of and reduce resource consumption of the tests.

See https://github.com/google/googletest/blob/master/googletest/docs/AdvancedGuide.md for more details.