Support `dolt_test_run()` on arbitrary refs

It would be useful if you could call `dolt_test_run()` on an arbitrary ref. This would allow you to run tests on branches other than the one currently checked out. Also, if you could run tests on any commit, you'd effectively be able to query for historical test run data.

For instance:
```sql
SELECT * FROM DOLT_TEST_RUN() AS OF 'some_other_ref'

```