build: test-driver.scm: Make output redirection optional.

This makes it easier (and less surprising) for users to experiment with the
custom Scheme test driver directly.  The behavior is unchanged from Automake's
point of view.

* build-aux/test-driver.scm (main): Make the --log-file and --trs-file
arguments optional and update doc.  Only open, redirect and close a port to a log file when
the --log-file option is provided.  Only open and close a port to a trs file
when the --trs-file option is provided.
(test-runner-gnu): Set OUT-PORT parameter default value to the current output
port.  Set the TRS-PORT parameter default value to a void port.  Update doc.