[WIP] Add test coverage instrumentation

[federicobond]

This is not working yet.

[maraoz]

@federicobond ping me when it's ready. In general, I prefix the PR title with "WIP" if it's not ready for merging, FYI

[federicobond]

I keep getting this error after all tests run:

Cleaning up...        

      There was an error generating coverage. Possible reasons include:                  
      1. Another application is using port 8555                                          
      2. Truffle crashed because your tests errored                                      

    Error: ENOENT: no such file or directory, open './allFiredEvents'                    
Exiting without generating coverage...