mktag tests: invert --no-strict test

Change the mktag --no-strict test to actually test success under
--no-strict, that test was added in 06ce79152b (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>

t/t3800-mktag.sh

@@ -33,6 +33,8 @@ check_verify_failure () {
 		then
 			test_must_fail git mktag <tag.sig 2>err2 &&
 			test_cmp err err2
+		else
+			git mktag --no-strict <tag.sig
 		fi
 	'
 }