Compiler: Fix fixture test comparison between cpp2 and mstch_cpp2
Compiler: Fix fixture test comparison between cpp2 and mstch_cpp2

Summary:
The original code to compare cpp2 and mstch_cpp2 generated files was flawed. Find code comment below for the error.

This modified code:
* Only makes the comparison when both cpp2 and mstch_cpp2 are marked to be generated.
* Compare that cpp2 and mstch_cpp2 have the same number of files and same file names.
* Compares content to make sure both have equal content.

Reviewed By: chadparry

Differential Revision: D4374898

fbshipit-source-id: 0a14081ac86b6af11c6cef95518c4cf140ecf3fa

SergeyMakarenko/fbthrift,facebook/fbthrift,facebook/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,facebook/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,getyourguide/fbthrift,facebook/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift