mktag tests: invert --no-strict test

mktag tests: invert --no-strict test

Change the mktag --no-strict test to actually test success under
--no-strict, that test was added in 06ce79152be (mktag: add a
--[no-]strict option, 2021-01-06).

It doesn't make sense to check that we have the same failure except
when we want --no-strict, by doing that we're assuming that the
behavior will be different under --no-strict, bun nothing was testing
for that.

We should instead assert that --strict is the same as --no-strict,
except in the cases where we've declared that it's not.

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