Add a basic test that the configured fallback hostname is OK

Ideally, assert_cc() would be used for this, so that it is not possible to even
compile systemd with something like '-Dfallback-hostname=.foo'. But to do a
proper check we need to call hostname_is_valid(), and we cannot depend on being
able to run code (e.g. during cross-compilation). So let's do a very superficial
check in meson, and a proper on in test-util.

src/test/test-hostname-util.c

@@ -140,6 +140,13 @@ static void test_read_etc_hostname(void) {
         unlink(path);
 }
 
+static void test_fallback_hostname(void) {
+        if (!hostname_is_valid(FALLBACK_HOSTNAME, false)) {
+                log_error("Configured fallback hostname \"%s\" is not valid.", FALLBACK_HOSTNAME);
+                exit(EXIT_FAILURE);
+        }
+}
+
 int main(int argc, char *argv[]) {
         log_parse_environment();
         log_open();
@@ -148,5 +155,7 @@ int main(int argc, char *argv[]) {
         test_hostname_cleanup();
         test_read_etc_hostname();
 
+        test_fallback_hostname();
+
         return 0;
 }