Allow developers to download and run only the tests that failed in CI.

[tpendragon]

Descriptive summary

Running the full test suite takes two hours, but CI runs them in ~ 12 minutes, so most developers push their branches to CI to run the whole suite. It would be good if we can download the tests that failed so we can run just those locally.

I think we can do this if the parallel jobs store spec/example.txt, and then combine those files. Then we can download the combination and run rspec spec --only-failures.

Rationale

Let's make it easy to run tests.

[tpendragon]

After a little research: Each parallel run does indeed generate its own spec/example.txt - they look like this:

example_id                                                                                 | status  | run_time        |
------------------------------------------------------------------------------------------ | ------- | --------------- |
./spec/controllers/hyrax/file_sets_controller_spec.rb[1:1:1:1:1]                           | passed  | 2.26 seconds    |
./spec/controllers/hyrax/file_sets_controller_spec.rb[1:1:2:1]                             | passed  | 2.1 seconds     |
./spec/controllers/hyrax/file_sets_controller_spec.rb[1:1:3:1:1]                           | passed  | 1.94 seconds    |
./spec/controllers/hyrax/file_sets_controller_spec.rb[1:1:3:2:1]                           | passed  | 2.03 seconds    |
./spec/controllers/hyrax/file_sets_controller_spec.rb[1:1:3:3:1]                           | passed  | 2.04 seconds    |
./spec/controllers/hyrax/file_sets_controller_spec.rb[1:1:3:4:1:1:1]                       | passed  | 3.32 seconds    |