Updates to Scalapack formula

- always run tests
- make openblas optional instead of recommended.

Testing scientific software shouldn't be optional.

The second change corrects a side-effect when another formula `depends_on` scalapack and also has an optional dependency on openblas. Those may conflict if openblas is `:recommended` in scalapack and `:optional` in the other formula. This is the case with Mumps (PR #392). Because the Accelerate framework is always there, it seems there's no reason for openblas to be `:recommended`.