.gitlab-ci.yml:clang-tidy: Enable BUILD_TESTING=ON
As we want to check the catch2 tests we need to enable BUILD_TESTING=ON so that the tests get added to the compile_commands.json generated by CMake.
This was missed when we added the catch2 tests to be checked by clang-tidy in: 91d814c7 (.gitlab-ci.yml: Run clang-tidy against cacth2 tests, 2024-02-09).