Add simple tests for Supply.stable

Unfortunately, some of them fail, so there is more work to be done

S17-supply/stable.t

@@ -0,0 +1,38 @@
+use v6;
+use lib 't/spec/packages';
+
+use Test;
+use Test::Tap;
+
+plan 20;
+
+for (ThreadPoolScheduler, CurrentThreadScheduler) {
+    $*SCHEDULER = .new;
+    isa_ok $*SCHEDULER, $_, "***** scheduling with {$_.gist}";
+
+    {
+        my $s = Supply.new;
+        tap_ok $s.stable(2),
+          [1,4],
+          ".stable worked",
+          :after-tap( {
+               $s.more(1);
+               sleep 1;
+               $s.more(1);
+               sleep 1;
+               $s.more(2);
+               $s.more(3);
+               sleep 1;
+               $s.more(4);
+               $s.done;
+          } ),
+          :live;
+    }
+
+    {
+        my $for   = Supply.for(1..10);
+        my $stable = $for.stable(0);
+        ok $for === $stable, "stable by 0 is a noop";
+        tap_ok $stable, [1..10], "noop stable";
+    }
+}