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`.
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