Fix spurious fails in Supply.interval tests

The cancel inside the CATCH block could also happen before the cancellation
value was set because of an exception.  And then bomb again, causing the test
failure.

src/core/Supply.pm

@@ -125,7 +125,7 @@ my class Supply {
                     {
                         state $i = 0;
                         emit($i++);
-                        CATCH { $cancellation.cancel }
+                        CATCH { $cancellation.cancel if $cancellation }
                     },
                     :every($!interval), :in($!delay)
                 );