t/lib-credential: use test_i18ncmp to check stderr

The credential tests have a "check" function which feeds some input to
git-credential and checks the stdout and stderr. We look for exact
matches in the output. For stdout, this makes sense; the output is
the credential protocol. But for stderr, we may be showing various
diagnostic messages, or the prompts fed to the askpass program, which
could be translated. Let's mark them as such.

t/lib-credential.sh

@@ -19,7 +19,7 @@ check() {
 		false
 	fi &&
 	test_cmp expect-stdout stdout &&
-	test_cmp expect-stderr stderr
+	test_i18ncmp expect-stderr stderr
 }
 
 read_chunk() {