Merge branch 'en/test-cleanup'

Test cleanup.

* en/test-cleanup:
  t6020: new test with interleaved lexicographic ordering of directories
  t6022, t6046: test expected behavior instead of testing a proxy for it
  t3035: prefer test_must_fail to bash negation for git commands
  t6020, t6022, t6035: update merge tests to use test helper functions
  t602[1236], t6034: modernize test formatting