Added TestResultLogger for customising test feedback.

1) When `test` is run and there are no tests available, omit logging output.
   Especially useful for aggregate modules. `test-only` et al unaffected. (#1185)

2) Added a new setting `testResultLogger` to allow customisation of logging of test results.
Merge pull request #1225 from japgolly/shhh_tests

Added setting 'testResultLogger' which allows customisation of test reporting.