Tests update.
* ps/ref-tests-update-more:
t6301: write invalid object ID via `test-tool ref-store`
t5551: stop writing packed-refs directly
t5401: speed up creation of many branches
t4013: simplify magic parsing and drop "failure"
t3310: stop checking for reference existence via `test -f`
t1417: make `reflog --updateref` tests backend agnostic
t1410: use test-tool to create empty reflog
t1401: stop treating FETCH_HEAD as real reference
t1400: split up generic reflog tests from the reffile-specific ones
t0410: mark tests to require the reffiles backend