Support parallel testing (parallel_tests)

[theomelo]

Is your feature request related to a problem? Please describe.

When running specs in parallel using parallel_tests it gets into a race condition to write the cache file. To understand what happens, download this repo and run parallel_rspec spec. Make sure you don't run the test suite directly with rspec before parallel, because it will cache the result and this problem will not happen.

Describe the solution you'd like

It should gracefully handle multiple instances of rspec-tracer writing the cache.

Describe alternatives you've considered

My original project is 8 years old and it's on Rails 5.2, so running the specs in parallel locally are only possible using parallel_specs. We want to take advantage of the caching system rspec-tracer provides to decrease the time we take running the full test suite.