Clean up IPV6 config checks in test_wsgi.

Two tests in test_wsgi require a configured IPV6 interface.
We added a check/skip for this but it wasn't very graceful and
didn't work on OS-X or platforms other than Linux.

This patch implements the checks a bit cleaner using the skip_if
decorator, and combines the linux/path check and configured interface
check.

Change-Id: I950fdfeb8135912faf5dcaabe52fb87abd73dd1e
Merge "Clean up IPV6 config checks in test_wsgi."