add PullPaymentCapable test

test/PullPaymentCapableExample.sol

@@ -0,0 +1,9 @@
+import '../contracts/PullPaymentCapable.sol';
+
+// Example class using PullPaymentCapable
+contract PullPaymentCapableExample is PullPaymentCapable {
+  // test helper function to call asyncSend
+  function callSend(address dest, uint amount) external {
+    asyncSend(dest, amount);
+  }
+}