Allow for repeated tests to run in parallel

[Badgerati]

Currently you can run tests repeatedly in serial using the [Repeat] attribute. With this issue I suggest adding the concept of being able to run repeated tests in parallel, maybe with a [ParallelRepeat] attribute, or add a parallel flag to the [Repeat] attribute:

[Repeat(3, Parallel = true)]

// or

[ParallelRepeat(3)]

Personally I prefer the first one.

[Badgerati]

After thinking about it, I'm going to use [ParallelRepeat], as this way it can only be used on Test methods, and not TestFixture classes. TestFixtures should not have their repeats run in parallel, in case any of the underlying Tests are required to be run sequentially.

[Badgerati]

There is now a new [ParallelRepeat(<value>)] attribute with which to decorate your tests.