Do not add tests if project is included via add_subdirectory/FetchContent

[traversaro]

We had a failure in a project that included serial_cpp via FetchContent, as it started to enable BUILD_TESTING, and we experience the failure:

CMake Error at /home/runner/micromamba/envs/internalprojectenv/share/cmake-4.0/Modules/FindPackageHandleStandardArgs.cmake:227 (message):
  Could NOT find GTest (missing: GTEST_LIBRARY GTEST_INCLUDE_DIR
  GTEST_MAIN_LIBRARY)
Call Stack (most recent call first):
  /home/runner/micromamba/envs/internalprojectenv/share/cmake-4.0/Modules/FindPackageHandleStandardArgs.cmake:591 (_FPHSA_FAILURE_MESSAGE)
  /home/runner/micromamba/envs/internalprojectenv/share/cmake-4.0/Modules/FindGTest.cmake:273 (find_package_handle_standard_args)
  build/_deps/serial_cpp-src/tests/CMakeLists.txt:2 (find_package)

The additional dependency on GTest if one enable BUILD_TESTING definitely breaks POLA.

A more user-friend behavior is to disable tests if PROJECT_IS_TOP_LEVEL is OFF, unless someone really needs them enabled, in that case the new advanced option serial_cpp_FORCE_RESPECT_BUILD_TESTING can be used to force them to be enabled even if PROJECT_IS_TOP_LEVEL is OFF.

[dariosortino]

Honestly way out of my knowledge but approving to ack the modification