Fix test_time_zone_from_posix_tz in environments without a tzdb

by specifying a POSIX TZ string instead of a time zone name.

src/offset/local/tz_info/timezone.rs

@@ -859,7 +859,7 @@ mod tests {
                 assert_eq!(time_zone_local, time_zone_local_1);
             }
 
-            let time_zone_utc = TimeZone::from_posix_tz("UTC")?;
+            let time_zone_utc = TimeZone::from_posix_tz("UTC0")?;
             assert_eq!(time_zone_utc.find_local_time_type(0)?.offset(), 0);
         }