Move gtest_add_tests to a new GoogleTest module

Separate the gtest_add_tests macro into a separate module. GTest or GoogleTest can be used by a project in a several different ways, including installed libraries in the system, from an ExternalProject, or adding the GTest source directory as a sub directory of the project. As not all of these uses are supported by the FindGTest module the useful gtest_add_tests macro is separated to easily enable reuse.

The addresses issues discussed in #14151 (closed)