Capture and suppress stdout and stderr if there is no error

**Description**
The python `pytest` framework only displays stdout and stderr if there is an error in the unit test. And then it displays each separately, not interleaved.

**Additional context**
It would be great if catch2 did this. #49 implies that it does, but it doesn't seem to.

The issue names no files, tests, or entry points. Start by reviewing Catch2's existing output handling and the behavior referenced by #49. Done means stdout and stderr are captured, suppressed for passing tests, and shown interleaved when a test errors.

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