Scalatest: BeforeAndAfterAll can be stacked

When using scalatest and spray-testkit, there may be multiple traits implementing BeforeAndAfterAll in a test.
To ensure that all of them are called, ScalatestInterface should call the super implementation. 
Merge pull request #222 from adamw/patch-1

test-kit: allow BeforeAndAfterAll stacking (scalatest)