write date output format logic with exhaustive tests

Also, exhausting tests.

Author: BenWiederhake

tests/coreutils.rs

@@ -13,6 +13,9 @@ mod basename;
 #[path = "coreutils/cat.rs"]
 mod cat;
 
+#[path = "coreutils/date.rs"]
+mod date;
+
 #[path = "coreutils/dd.rs"]
 mod dd;