Add new test file for supply syntax non-blocking.

We'll flesh out the semantics of `await` inside of a `supply`,
`react`, and `whenever` block a little later on. For now, this just
ensures that `react` itself is non-blocking.

S17-supply/syntax-nonblocking-await.t

@@ -0,0 +1,32 @@
+use v6.d.PREVIEW;
+use Test;
+
+plan 1;
+
+{
+    # Start 100 workers that do a `react`. This will, if `react` blocks,
+    # clog up the thread pool.
+    my $sup = Supplier.new;
+    my $c = Channel.new;
+    for ^100 {
+        start {
+            react {
+                whenever $sup {
+                    $c.send($_);
+                    done;
+                }
+                $c.send("started");
+            }
+        }
+    }
+
+    # Ensure they're all started.
+    $c.receive xx 100;
+
+    # Start one more worker that will provide a value for all of the
+    # reacts. If the thread pool is clogged, it won't get to run.
+    await start { $sup.emit(2) }
+
+    # Should now get all values sent.
+    is [+](($c.receive xx 100)), 200, 'start react { ... } is non-blocking';
+}