Preload assets before backend specs

[jhawthorn]

We've been seeing some builds fail with "Request failed to reach server" on the first request.

This happens because we have quite a few assets and it can take some time to compile them. Since this was done on the first request, this sometimes caused poltergeist to error.

This commit gets sprockets-rails to preload the assets before the suite is run (if there is a feature spec included in the run).

This has the added advantage of not including the time to precompile specs as part of the first spec's run time. Running rspec --profile=10 should now be more useful in showing actually slow specs.

Alternative to #2116

[cbrunsdon]

Looks like a nice solution

[jhawthorn]

Before

$ rspec --profile=10 ./spec/features/admin/reports_spec.rb

Randomized with seed 54973
..

Top 2 slowest examples (15.09 seconds, 99.5% of total time):
  Reports visiting the admin reports page should have the right content
    14.61 seconds ./spec/features/admin/reports_spec.rb:7
  Reports searching the admin reports page should allow me to search for reports
    0.48311 seconds ./spec/features/admin/reports_spec.rb:49

Finished in 15.17 seconds (files took 2.36 seconds to load)
2 examples, 0 failures

Randomized with seed 54973

After

$ rspec --profile=10 ./spec/features/admin/reports_spec.rb --seed=54973

Randomized with seed 54973
..

Top 2 slowest examples (0.89705 seconds, 5.9% of total time):
  Reports searching the admin reports page should allow me to search for reports
    0.57514 seconds ./spec/features/admin/reports_spec.rb:49
  Reports visiting the admin reports page should have the right content
    0.32191 seconds ./spec/features/admin/reports_spec.rb:7

Finished in 15.27 seconds (files took 2.35 seconds to load)
2 examples, 0 failures

Randomized with seed 54973

Time for the suite hasn't changed (the ~14 seconds to load assets is just moved earlier), but the first specs run in a more similar time to the other specs in the suite and won't error waiting on assets to compile.

[mamhoff]

This seems great!

[tvdeyen]

👌🏻

[kennyadsl]

🥇

[mtomov]

I will happily steal this right off the bat into our projects, as I've struggled with this for quite a while now with no good solution on the horizon. Thanks a lot!