Merge branch 'hn/refs-test-cleanup'

A handful of tests that assumed implementation details of files
backend for refs have been cleaned up.

* hn/refs-test-cleanup:
  t6001: avoid direct file system access
  t6500: use "ls -1" to snapshot ref database state
  t7064: use update-ref -d to remove upstream branch
  t1410: mark test as REFFILES
  t1405: mark test for 'git pack-refs' as REFFILES
  t1405: use 'git reflog exists' to check reflog existence
  t2402: use ref-store test helper to create broken symlink
  t3320: use git-symbolic-ref rather than filesystem access
  t6120: use git-update-ref rather than filesystem access
  t1503: mark symlink test as REFFILES
  t6050: use git-update-ref rather than filesystem access