Add knapsack qa report and use it to run tests in parallel

Use the RSpec runner with knapsack

The way the Knapsack runner uses exec to start rspec seems
incompatible with the way we expect it to work. Plus, it requires
specifying KNAPSACK_TEST_DIR.

Instead, we use knapsacks AllocatorBuilder to select the spec
files to run, and then start rspec as normal, via
RSpec::Core::Runner.run

This also means we can incorporate tags.

Let the job run automatically

Include KNAPSACK_TEST_FILE_PATTERN in vars

Check all defined knapsack env vars before requiring knapsack

This pulls in a fix for libxslt that addresses CVE-2019-11068:
https://github.com/sparklemotion/nokogiri/releases

Does not retry if tagged :quarantine. Also adds unit tests.

This will allow developers to run `bundle install` on both directories
and avoid additional calls to RubyGems for local development. Also sets
up the possibility of improved caching as mentioned in
https://gitlab.com/gitlab-org/gitlab-ce/issues/55843.

* upstream/master: (106 commits)
  Remove callback as we already update accordingly on services
  added missing imports
  correctly imports performance bar
  Added imports for dispatcher routes
  Make the exposing of the Application secret more explicit
  Add documentation about when the application API was added
  Add application create API
  Return a blank JSON response for a missing .js file to prevent Rails CSRF errors
  add CHANGELOG.md entry for !15804
  disable CopyAsGFM on iOS due to bug in webkit
  Update changelog
  GitalyClient::ConflictsService#conflicts? should return true for conflicts with missing side
  Return more consistent values for merge_status on MR API
  Fix a migration spec messing up the MergeRequestDiff DB schema
  Backport changes to Gitlab::Checks::ChangeAccess from EE
  remove webpack bundle tag for monitoring:environments:metrics
  Set timezone for karma to UTC
  Upgrade jasmine + raven to newer versions
  Ensure Gitaly Ruby gems are installed using the correct Gemfile and at the correct location
  Clarify that a feature that isn't in review by the 1st or 3rd doesn't necessarily miss the freeze
  ...