Add some newlines and a comment to improve readability.
Use local string port instead of parameterizing current-output-port.
Pass symbol 'BEGIN to tell test reporter that evaluation of a test
starts. Adapt documentation of current-test-reporter accordingly.
Use define-opt instead of case-lambda.
Fix standard test runner so that its output is properly indented and
lines are properly terminated.
Refactor standard test runner so that it is possible to plug in another
runner with different output.
