reduce eyes parallelism from 79 to 20 to deflake drone

[davidsbailey]

Sauce Labs is dropping our concurrency from 200 to 65 soon. As part of the work to move some of our UI test load to device farm, I noticed that our DTT currently maxes out our current Sauce Labs concurrency, using 120 + 79 = 199 at the beginning of the DTT:

• Starting 738 dashboard UI tests in 120 threads...
• Starting 79 dashboard Eyes tests in 172 threads... (this is 79x not 172x parallel)

We also use saucelabs during CI, but we use saucelabs priority option to give higher priority to DTT:

• DTT:

  code-dot-org/lib/rake/test.rake

  Line 46 in 7e49f6f

  '--priority 0'

• CI:

  code-dot-org/dashboard/test/ui/runner.rb

  Line 115 in 7e49f6f

  options.priority = '99'

Saucelabs docs explain that pending jobs with lower priority number are always selected before jobs with higher priority number. This could be creating a situation where saucelabs is entirely unavailable for CI jobs for a period of minutes at the start of each DTT.

Therefore this PR accomplishes 2 goals:

• start preparing us for the upcoming drop in saucelabs concurrency
• see if we can fix some hard-to-diagnose issues in drone

Speculation

This might explain why we have UI tests failing in drone with the following mysterious errors:

• https://cucumber-logs.s3.amazonaws.com/circle/50659/Chrome_teacher_tools_level_types_free_response_contained_levels_output.html?versionId=nP3abYYAshFDeO6xPs6Vn91A1IqJkFyO
  • Misconfigured -- No active tunnel found for provided identifier (Selenium::WebDriver::Error::SessionNotCreatedError)
• https://drone.cdn-code.org/code-dot-org/code-dot-org/50416/2/5
  https://cucumber-logs.s3.amazonaws.com/circle/50416/Chrome_teacher_tools_teacher_student_toggle_eyes_output.html?versionId=ut_wgt3UCM4kyeH4Je_qUEbtqNOD1.iK
  • The test session has already finished, and can't receive further commands. For help please check https://docs.saucelabs.com/dev/error-messages (Selenium::WebDriver::Error::WebDriverError)

This is highly speculative and may be hard to verify, but it seems worth a shot at potentially fixing this class of errors.

Testing story

I patched this change into the test machine, and found that it will not have any impact on DTT runtimes, because eyes tests still finish faster than regular UI tests.

before:

79 passed. 0 failed. Test count: 79. Duration: 9:39 minutes. Total successful reruns of flaky tests: 0.
738 passed. 0 failed. Test count: 738. Duration: 29:45 minutes. Total successful reruns of flaky tests: 5.

after:

78 passed. 1 failed. Test count: 79. Duration: 17:09 minutes. Total successful reruns of flaky tests: 0.
741 passed. 0 failed. Test count: 741. Duration: 30:17 minutes. Total successful reruns of flaky tests: 2.

Deployment notes

Deploy independently of any Device Farm changes to help reduce drone flakiness