That is why I have been suggesting that we provide a test operation that binds a stream --- because most of the test frameworks I have worked with provide a test report, rather than returning results.
fyi, stefil returns a CLOS object containing the test results (and provides slime inspector customizations to present it specially in the inspector).
as it doesn't work too well through asdf:test-op, it also provides a print-object method that prints a minimal text representation of the data.
when test-op'ed, we print the result object to *standard-output*. when used interactively, the test defun that was used to start the testing simply returns the result value which we inspect in slime when needed.