tests: Add googletest

Add googletest via a new CMake module, TestTools. We created TestTools
so that googletest and fff or other unit testing tools can be described
to CMake before other software in the "components" folder. This allows
any component to use googletest from its own testing folder, enabling
the possibility to write unit tests for any component glue code.

Additionally, add a helper function for tests to use to ensure that
googletest is excluded from our clang-tidy checks.

Signed-off-by: Jaeden Amero <jaeden.amero@arm.com>