modernize t9300: use test_must_fail
modernize t9300: use test_must_fail

One test case open-codes a test for an expected failure. Replace it by
test_must_fail.

Signed-off-by: Johannes Sixt <fd432345609ae4d58c1aa629cb66ba96d1391622@kdbg.org>
Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git