import/export unittest: Improve logging and fix one race condition

Apart from improved logging, one race condition is fixed. If
the destination's status file became available, the port would
be returned immediately, even if it was still “None”. Most of
the time it worked, but not always. Now an additional check
ensures the port evaluates to True.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>