Allow for parallel processing of tests

[oubiwann]

Talked with @indiejames about this, based on his experiences with running the test suite in CMR:

• enable parallel by default
• namespaces should probably be the lowest level of granularity for parallel execution
• explore using metadata for overriding parallel execution (I think he said this is what eftest does?)

Some other nice things to consider:

• preserve ordering of tests + assertions superficially (indexing before execution)
• display results in indexed order using core.async for batching display (fun! :-) )