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