Merge branch 'ps/t-fixes-for-git-test-long' into next

Various test scripts have been updated to clean up large temporary files and repositories, reducing peak disk usage during testing. Also, expensive tests have been disabled on platforms that lack sufficient resources (like 32-bit platforms and Windows CI runners), and the long test suite has been enabled in GitLab CI. * ps/t-fixes-for-git-test-long: gitlab-ci: enable "GIT_TEST_LONG" gitlab-ci: disable RAM disk on macOS jobs t: use `test_bool_env` to parse GIT_TEST_LONG t7900: clean up large EXPENSIVE repository t7508: skip EXPENSIVE test that is broken without SIZE_T_IS_64BIT t5608: reduce maximum disk usage t4141: fix inefficient use of dd(1) t0021: skip EXPENSIVE test that is broken without SIZE_T_IS_64BIT README: add GitLab CI badge to make it more discoverable