Fix a twisted test to work with Twisted 12.3.0 or better.

Twisted has updated trial to work better with unittest's test runner.
As a result, skipped tests are no longer reported as failures.  Since
this is the desired result, fix the test to no longer expect a failure
for the skipped test.  This was tested from Twisted 10.0 to 12.3.
Merge pull request #640 from jszakmeister/fix-test-with-newer-twisted

Fix a twisted test to work with Twisted 12.3.0 or better.