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

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

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