Provide sharding support

[horenmar]

Sharding is useful trick for running tests in parallel that can be done with very minimal understanding between the test runner and the test framework.

With Catch2, you can run tests in parallel like this:

./tests --shard-index 0 --shard-count 3 &
./tests --shard-index 1 --shard-count 3 &
./tests --shard-index 2 --shard-count 3 &
wait

this splits all tests into three equi-sized groups and runs them in parallel. (Using the default decl order, but other orders are supposed as well)

[cschreib]

I think this is likely to be a desirable feature; I'd expect IDEs to make use of this for parallel test runs. It is much easier to implement than #33 at any rate! Will take a look, thanks for the suggestion.