Uses the parallel_tests gem to execute tests in multiple processes
simultaneously on the same machine.

Adds the `--parallel` CLI option that instructs the QA framework
to use the parallel_tests executable.

Tests need access to global state contained in `Runtime::Scenario`
so when `--parallel` is invoked `Runtime::Scenario` is serialized
to an environment variable, which is passed to parallel_tests,
and then deserialized in `spec_helper`.

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.