speed up db/indextest by 5 minutes (#6911)

- create TestBucketPoolEnsureNoIndexes to do:
    - dcp purge deletion instead of flush
    - all index deletion
- shorten the number of test that need to recreate indexes in index
  tests
    - parametrize QueryAllRoles/QueryRoles inside existing case
    - put ExplainQuery tests as subtests of existing test, so we don't create
      indexes just to test covered query
    - put includeDeleted subtest inside database creation