Use skipTest from testtools instead of inherited Exception

SkipTest is raised if there is no func_test section in the Swift test
config file. However, the currently raised Exception will result in a
failed test, not in a test marked as skipped.

Since we already use testtools we can use the included skipTest easily.

Change-Id: I1bbb1f9dbe31fb0698d774550708d1196b266625
Merge "Use skipTest from testtools instead of inherited Exception"