mailmap tests: improve --stdin tests

mailmap tests: improve --stdin tests

The --stdin tests setup the "contact" file in the main setup, let's
instead set it up in the test that uses it.

Also refactor the first test so it's obvious that the point of it is
that "check-mailmap" will spew its input as-is when given no
argument. For that one we can just use the "expect" file as-is.

Also add tests for how other "--stdin" cases are handled, e.g. one
where we actually do a mapping.

For the rest of --stdin testing we just assume we're going to get the
same output. We could follow-up and make sure everything's
round-tripped through both --stdin and the file/blob backends, but I
don't think there's much point in that.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>