Only show cider-test-report buffer on test failures.

This PR doesn't show the *cider-test-report* on successful tests anymore. Most
people will close that buffer anyway right after it popped up, and a message in
the echo area is probably enough to indicate that everything is fine. Setting
`cider-test-report-on-success` to `t` restores the old behaviour.
`plusp` is in `cl-lib`.