mktag tests: test "hash-object" compatibility

mktag tests: test "hash-object" compatibility

Change all the successful "mktag" tests to test that "hash-object"
produces the same hash for the input, and that fsck passes for
both.

This tests e.g. that "mktag" doesn't trim its input or otherwise munge
it in a way that "hash-object" doesn't.

Since we're doing an "fsck --strict" here at the end let's incorporate
the creation of the "mytag" name into this test, removing the
special-case at the end of the file.

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