webm2pes: Begin addition of tests.

Just the basics:
- Some new test utilities.
- Basic setup for webm2pes testing.

Change-Id: I16bf0f5ef36e7c01f2788b2c92600d6a936bbd40

webm2pes.cc

@@ -302,6 +302,7 @@ bool Webm2Pes::ConvertToFile() {
     cluster = webm_parser_->GetNext(cluster);
   }
 
+  std::fflush(output_file_.get());
   return true;
 }