tests: Add a help message for the test runner

In case we forget the name of the test case or typo it, the test runner
will now list the test cases in the test binary.

tests/test-runner.c

@@ -89,6 +89,24 @@ find_test(const char *name)
 	return NULL;
 }
 
+static void
+usage(const char *name, int status)
+{
+	const struct test *t;
+
+	fprintf(stderr, "Usage: %s [TEST]\n\n"
+		"With no arguments, run all test.  Specify test case to run\n"
+		"only that test without forking.  Available tests:\n\n",
+		name);
+
+	for (t = &__start_test_section; t < &__stop_test_section; t++)
+		fprintf(stderr, "  %s\n", t->name);
+
+	fprintf(stderr, "\n");
+
+	exit(status);
+}
+
 static void
 run_test(const struct test *t)
 {
@@ -119,11 +137,14 @@ int main(int argc, char *argv[])
 
 	leak_check_enabled = !getenv("NO_ASSERT_LEAK_CHECK");
 
+	if (argc == 2 && strcmp(argv[1], "--help") == 0)
+		usage(argv[0], EXIT_SUCCESS);
+
 	if (argc == 2) {
 		t = find_test(argv[1]);
 		if (t == NULL) {
 			fprintf(stderr, "unknown test: \"%s\"\n", argv[1]);
-			exit(EXIT_FAILURE);
+			usage(argv[0], EXIT_FAILURE);
 		}
 
 		run_test(t);