Testcases and benchmarks are rather different entities. You won't usually want
to run benchmarks in the same environment you are wanting to run tests in,
so this feature allows to differentiate between the two.

We also add a "benchmark" make target (similar to check), which runs all
configured benchmarks.

Change-Id: I33759ce44c34e42a6a3a88f34e7b9c4372380721
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>