Run tests in parallel via parallel_tests

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`.