Add option to suppress test output

[Symbolics]

When testing numeric functions, oftentimes many thousands of tests will be run against a reference implementation. Under these testing scenarios, having 'x' and 'f' printed for each test isn't useful and a summary at the end would be more helpful.

[fjl]

You can hide it with

  (let ((*test-dribble* (make-broadcast-stream)))
     ...test code...)