test: add known broken test for bad utf8 in config

We should ideally print an informative error message, but at the very
least we should not exit with success.

test/T030-config.sh

@@ -3,6 +3,8 @@
 test_description='"notmuch config"'
 . $(dirname "$0")/test-lib.sh || exit 1
 
+cp notmuch-config initial-config
+
 test_begin_subtest "Get string value"
 test_expect_equal "$(notmuch config get user.name)" "Notmuch Test Suite"
 
@@ -193,4 +195,10 @@ test_begin_subtest "get built_with.nonexistent prints false"
 output=$(notmuch config get built_with.nonexistent)
 test_expect_equal "$output" "false"
 
+test_begin_subtest "Bad utf8 reported as error"
+test_subtest_known_broken
+cp initial-config bad-config
+printf '[query]\nq3=from:\xff\n' >>bad-config
+test_expect_code 1 "notmuch --config=./bad-config config list"
+
 test_done