Add tests for Supply.delay

We seem to have a problem with the ThreadPool scheduler case :-(

S17-supply/delay.t

@@ -0,0 +1,28 @@
+use v6;
+use lib 't/spec/packages';
+
+use Test;
+use Test::Tap;
+
+plan 22;
+
+for (ThreadPoolScheduler, CurrentThreadScheduler) {
+    $*SCHEDULER = .new;
+    isa_ok $*SCHEDULER, $_, "***** scheduling with {$_.gist}";
+
+    {
+        my $delay = 2;
+        my $now   = now;
+        my $seen;
+        tap_ok $now.Supply.delay($delay).do( { $seen = now } ),
+          [$now], ".delay worked";
+        ok $seen && $seen >= $now + $delay, "supply sufficiently delayed";
+    }
+
+    {
+        my $for   = Supply.for(1..10);
+        my $delay = $for.delay(0);
+        ok $for === $delay, "delaying by 0 is a noop";
+        tap_ok $delay, [1..10], "noop delay";
+    }
+}