Add some DateTime subclassing tests

S32-temporal/DateTime.t

@@ -1,7 +1,7 @@
 use v6;
 use Test;
 
-plan 247;
+plan 268;
 
 my $orwell = DateTime.new(year => 1984);
 
@@ -672,3 +672,20 @@ is ds("2016-02-29T00:00:00").later(:1year), "2017-02-28T00:00:00Z",
     class BarDate is DateTime does Foo {}
     is BarDate.now.a, [7,8,9], 'did role attributes get initialized ok';
 }
+
+{
+    class FooDateTime is DateTime { has $.foo };
+    for (2016,2,20,21,53,7),
+        '2016-02-20T21:53:07',
+        \(:2016year,:2month,:20day,:21hour,:53minute,:7second)
+    -> $datetime {
+        my $fdt = FooDateTime.new(|$datetime, foo => 42);
+        is $fdt.year, 2016, "is year in FooDateTime ok";
+        is $fdt.month,   2, "is month in FooDateTime ok";
+        is $fdt.day,    20, "is day in FooDateTime ok";
+        is $fdt.hour,   21, "is hour in FooDateTime ok";
+        is $fdt.minute, 53, "is minute in FooDateTime ok";
+        is $fdt.second,  7, "is second in FooDateTime ok";
+        is $fdt.foo,    42, "is foo in FooDateTime ok";
+    }
+}