Make it so that we can console.log in browser to debug gentest (#1718)

Summary:

Our gentest works by console.logging the contents of the test it is generating to the browser powered by the driver. The driver then reads the logs and writes it to a file. An unfortunate side effect here is that we cannot console.log to debug how the gentest logic actually works since the driver is expecting formatted code. To get around this I had the driver filter out logs with a certain prefix and add that a helper that logs a message with this prefix to the scripts.

Reviewed By: NickGerleman

Differential Revision: D64011035

gentest/test-template.html

@@ -8,6 +8,7 @@
   <script src="gentest-cpp.js"></script>
   <script src="gentest-java.js"></script>
   <script src="gentest-javascript.js"></script>
+  <script src="gentest-log.js"></script>
 
   <style>
     @font-face {