Fix: merge tap tests stdout and stderr

This makes the output and error statement ordered in the log
file and ensure that the first line is the tap test plan. Some tap
parser are confused if the test plan is not on the first line.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>