Add a trivial function in core test

To ensure that we don't accidentaly not verify that chrono compiles for core.

ci/core-test/src/lib.rs

@@ -1 +1,7 @@
 #![no_std]
+
+use chrono::{TimeZone, Utc};
+
+pub fn create_time() {
+    let _ = Utc.ymd(2019, 1, 1).and_hms(0, 0, 0);
+}