test: add known broken subtest for the bad config error message

This is a bit fragile w.r.t. glib changing their error message, but it
already helped me find one formatting bug, so for now I think it's
worth it, instead of just grepping for "UTF-8".

test/T030-config.sh

@@ -200,4 +200,13 @@ cp initial-config bad-config
 printf '[query]\nq3=from:\xff\n' >>bad-config
 test_expect_code 1 "notmuch --config=./bad-config config list"
 
+test_begin_subtest "Specific error message about bad utf8"
+test_subtest_known_broken
+notmuch --config=./bad-config config list 2>ERRORS
+cat <<EOF > EXPECTED
+GLib: Key file contains key “q3” with value “from:<3A>” which is not UTF-8
+Error: unable to load config file.
+EOF
+test_expect_equal_file EXPECTED ERRORS
+
 test_done