Group reffiles tests

This series groups REFFILES specific tests together. These tests are currently grouped together across the test suite based on functionality. However, since they exercise low-level behavior specific to the backend being used (in these cases, the ref-files backend), group them together based on which refs backend they test. This way, in the near future when the reftables backend gets upstreamed we can add tests that exercise the reftables backend close by in the t06xx area.

These patches also remove the REFFILES prerequisite, since all the tests in t06xx are reffiles specific. In the near future, once the reftable backend is upstreamed, all the tests in t06xx will be forced to run with the reffiles backend.

fixes: #199 (closed)