Merge branch 'jc/reffiles-tests'

Tests on ref API are moved around to prepare for reftable.

* jc/reffiles-tests:
  t5312: move reffiles specific tests to t0601
  t4202: move reffiles specific tests to t0600
  t3903: make drop stash test ref backend agnostic
  t1503: move reffiles specific tests to t0600
  t1415: move reffiles specific tests to t0601
  t1410: move reffiles specific tests to t0600
  t1406: move reffiles specific tests to t0600
  t1405: move reffiles specific tests to t0601
  t1404: move reffiles specific tests to t0600
  t1414: convert test to use Git commands instead of writing refs manually
  remove REFFILES prerequisite for some tests in t1405 and t2017
  t3210: move to t0601