GoogleTest: Fix include path in the generated file

The generated …_include.cmake file contained an inclusion of GoogleTestAddTests without any path. In general, this is a good approach since it enables to correctly catch possibly user-customized files.

However, in this case, it didn’t work this way since the …_include.cmake file is evaluated by a separate CMake call under a custom command. Because of this, the CMAKE_MODULE_PATH is not set as expected by the user and the said inclusion catches CMake own version of the module rather than the possibly user-customized one.

This change fixes this by making the inclusion with an absolute path determined upon the …_include.cmake file generation.

Topic-rename: GoogleTest-fix-generated-include