Support building python bindings when building unit tests is manually disabled. (#6771)
* If unit tests are manually excluded via `--cmake_extra_defines onnxruntime_BUILD_UNIT_TESTS=OFF` (e.g. testing changes to binary size where you want to keep the build time as quick as possible) it should still be possible to create the python bindings.
Update CMakeLists.txt to decouple the inclusion of onnxruntime_python.cmake from unit tests being enabled.
Update onnxruntime_python.cmake so it works when unit tests are disabled. Also skip copying of test py files when unit tests are disabled.