Add FailureListFormatter that logs parsable failures

[RohanM]

We'd like to keep track of intermittently failing specs, and to do that we'd like a parsable list of failing specs. Later versions of rspec contain FailureListFormatter that can do this, so port in a copy and add it as a default formatter, outputting to log/rspec-failures.log.

[nickbrowne]

After rebasing on master, looks like it does the trick 👍

$ bundle exec rspec-queue spec
..
  1) RSpecQueue::Configuration#worker_count RSPEC_QUEUE_WORKERS env var set fails
     Failure/Error: expect(true).to eq false
     
       expected: false
            got: true
     
       (compared using ==)
     # ./spec/rspec_queue/configuration_spec.rb:25:in `block (4 levels) in <top (required)>'
     # ./lib/rspec_queue/worker_runner.rb:22:in `run_specs'

Finished in 0.31236 seconds
(files took 0.10015 seconds to load)
2 examples, 1 failure
Top 5 slowest examples:
./spec/rspec_queue/configuration_spec.rb:12 took 0.01s, impact on build time is 0.0s (0.6%)
./spec/rspec_queue/configuration_spec.rb:20 took 0.0s, impact on build time is 0.0s (0.0%)

$ cat log/rspec-failures.log 
./spec/rspec_queue/configuration_spec.rb:24:fails