Write to stdout and stderr separately

Hi, I configured a Slurm job with err & out files, but only the err file had output written after running the CWL Conformance tests (https://github.com/kinow/cwl-mpi/issues/20).

I think it's because we are using the Python `StreamHandler`, https://github.com/common-workflow-language/cwltest/blob/9a355204c6f726344fb17a61c845f9f75b39a456/src/cwltest/__init__.py#L11, without specifying the stream to write to.

> Returns a new instance of the StreamHandler class. If stream is specified, the instance will use it for logging output; otherwise, sys.stderr will be used.

It's a harmless issue, but it would still be nice to organize the log.info / error output separately for some users.