t06xx: always execute backend-specific tests

The tests in t06xx exercise specific ref formats. Next to probing some basic functionality, these tests also exercise other low-level details specific to the format. Those tests are only executed though in case `GIT_TEST_DEFAULT_REF_FORMAT` is set to the ref format of the respective backend-under-test. Ideally, we would run the full test matrix for ref formats such that our complete test suite is executed with every supported format on every supported platform. This is quite an expensive undertaking though, and thus we only execute e.g. the "reftable" tests on macOS and Linux. As a result, we basically have no test coverage for the "reftable" format at all on other platforms like Windows. Adapt these tests so that they override `GIT_TEST_DEFAULT_REF_FORMAT`, which means that they'll always execute. This increases test coverage on platforms that don't run the full test matrix, which at least gives us some basic test coverage on those platforms for the "reftable" format. This of course comes at the cost of running those tests multiple times on platforms where we do run the full test matrix. But arguably, this is a good thing because it will also cause us to e.g. run those tests with the address sanitizer and other non-standard parameters. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>