localspl/tests: Improve the tests in test_XcvDataPort_AddPort by properly checking return values and avoiding a duplicat

[Gerald Pfeifer]

On Sun, 9 May 2010, testbot at testbot.winehq.org wrote:
> === WINEBUILD (build) === Patch failed

The issue was that a previous patch of mine (which got accepted)
conflicted and my local tree was not right.  Fixed and resubmitted,
testing at Wine Test Bot went well now:

  https://testbot.winehq.org/JobDetails.pl?Key=1975

Gerald