partrt: test: Fix print for stdout + stderr
partrt: test: Fix print for stdout + stderr

One of the two variables may be None, in which case an exception
is raised not being able to concatenate None and str.

Avoid this by printing a tuple consisting of (stdout, stderr).

Signed-off-by: Alexandru Avadanii <046230ef61405fd997a5cbd7cfcbca66da78c41e@enea.com>

OpenEneaLinux/rt-tools,OpenEneaLinux/rt-tools,OpenEneaLinux/rt-tools