Extract the `gtest_add_tests` macro from `FindGTest` 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.
Issue: #14151
