Feature: Consider adding batching

When tests have a large amount of overhead, and the test itself is fast, it would be nice to use batching to run multiple tests for each task.

In my use case it takes about 2+ seconds to start testing, and we have ~400 tests, each taking .4 seconds to run.

Running with 1 test per task it only results in a 2x speed up. Being able to batch to 8 tasks should give an approx 10x speedup.

I would just implement it as a parameter like --batch_size=8