test: don't go through all time zones if slow tests are disabled

Let's skip going through all locally available time zones if we're built
with slow tests disabled, as that's quite slow, but do at least one test
with the UTC zone.

src/test/test-time-util.c

@@ -448,6 +448,11 @@ static void test_format_timestamp_with_tz_one(const char *tz) {
 TEST(FORMAT_TIMESTAMP_with_tz) {
         _cleanup_strv_free_ char **timezones = NULL;
 
+        test_format_timestamp_with_tz_one("UTC");
+
+        if (!slow_tests_enabled())
+                return (void) log_tests_skipped("slow tests are disabled");
+
         assert_se(get_timezones(&timezones) >= 0);
         STRV_FOREACH(tz, timezones)
                 test_format_timestamp_with_tz_one(*tz);
@@ -896,6 +901,11 @@ static void test_parse_timestamp_with_tz_one(const char *tz) {
 TEST(parse_timestamp_with_tz) {
         _cleanup_strv_free_ char **timezones = NULL;
 
+        test_parse_timestamp_with_tz_one("UTC");
+
+        if (!slow_tests_enabled())
+                return (void) log_tests_skipped("slow tests are disabled");
+
         assert_se(get_timezones(&timezones) >= 0);
         STRV_FOREACH(tz, timezones)
                 test_parse_timestamp_with_tz_one(*tz);