t3210: move to t0601

t3210: move to t0601

Move t3210 to t0601, since these tests are reffiles specific in that
they modify loose refs manually. This is part of the effort to
categorize these tests together based on the ref backend they test. When
we upstream the reftable backend, we can add more tests to t06xx. This
way, all tests that test specific ref backend behavior will be grouped
together.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>