t: use test_might_fail() instead of manipulating exit code manually

t: use test_might_fail() instead of manipulating exit code manually

These tests manually coerce the exit code of invoked commands to
"success" when they don't care if the command succeeds or fails since
failure of those commands should not cause the test to fail overall.
In doing so, they intentionally break the &&-chain. Modernize by
replacing manual exit code management with test_might_fail() and a
normal &&-chain.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>