Encapsulate the whole "test detection" process into a Ruby class that's tested

From the following discussion.

The test case should actually be for tooling/bin/js_to_system_specs_mappings if we'd want to catch this in the future. At this point, I think we should create a new Ruby class that would encapsulate the whole "test detection" process, i.e.

        tooling/bin/find_tests ${RSPEC_CHANGED_FILES_PATH} ${RSPEC_MATCHING_TESTS_PATH};
        tooling/bin/js_to_system_specs_mappings ${RSPEC_CHANGED_FILES_PATH} ${RSPEC_MATCHING_TESTS_PATH};
        tooling/bin/find_changes ${RSPEC_CHANGED_FILES_PATH} ${RSPEC_MATCHING_TESTS_PATH} ${FRONTEND_FIXTURES_MAPPING_PATH};
        filter_rspec_matched_foss_tests ${RSPEC_MATCHING_TESTS_PATH} ${RSPEC_MATCHING_TESTS_FOSS_PATH};
        filter_rspec_matched_ee_tests ${RSPEC_MATCHING_TESTS_PATH} ${RSPEC_MATCHING_TESTS_EE_PATH};
        tooling/bin/view_to_js_mappings ${RSPEC_CHANGED_FILES_PATH} ${RSPEC_MATCHING_JS_FILES_PATH};

so that we reduce the risk of mistakes and we write tests for it.