Run tests in parallel

[sol]

Open issues:

• We need some synchronization mechanism to make sure that global resources are
  only accessed from one test at a time (see e.g. https://github.com/sol/setenv/blob/master/test/System/SetEnvSpec.hs#L77)
• silently does not work as expected, when used concurrently

[adinapoli]

Hello there.
I've noticed the same issue rising when I've tried to run a spec where its functions where having contention on a log file. We can take inspiration from Scala's specs2, where you can toggle parallel execution with the function "sequential" at the beginning of each spec. Here we could do something like :

mySpec  = hspec $ sequentially $ do
                    ....

[sol]

Yes, sounds like a good idea.

We would then probably run all specs that are marked with sequentially in the applications main thread (mainly because the runner/formatter also access stdout/stderr).

[sol]

For now specs have to be explicitly marked with parallel for parallel execution. That way it's backward compatible. In the future we can improve on this by:

• Adding a --parallel option to hspec-discover
• Providing a sequential combinator; if sequential and parallel are used in a nested way the innermost combinator should take precedence

[adinapoli]

Nice!