Cleaning up Super Scaffolding Partial Tests

[gazayas]

At first I set out to simply clean up the test files that were being generated in tmp/storage whenever running the Super Scaffolding TestFile test, but it turned out there was some more cleanup to get taken care of, so here are the different things I addressed.

Changes

1. Remove the foo.txt file that’s generated whenever we run super_scaffolding_system_test.rb
2. Move all the necessary logic to PartialTest and delete TestFile (I wrote TestFile before PartialTest was merged in, so these were separated before)
3. Make sure PartialTest runs in parallel

I made sure PartialTest ran in parallel in #338, but I figured there would be conflicts so I just decided to take care of it here.

[andrewculver]

@gazayas Can you resolve the conflict here?

[gazayas]

Closing in favor of #654.