t1400: avoid "test" string comparisons

t1400: avoid "test" string comparisons

Using the shell "test" here is inflexible, because we can't easily swap
it out for an i18n-aware version like we can with test_cmp and
test_i18ncmp. And it's not even saving us any processes, since we have
to use "cat" to get the output. So let's switch to using test_cmp, which
has the added bonus that it will produce better output if there's a
failure.

Note that not all of the changed outputs here are candidates for
translation, but I've converted all of them for consistency and to
benefit from the better output.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>