Replace skip with skipTest

testtools 2.8.0 removed TestCase.skip in favour of TestCase.skipTest
[1]. We were mainly using the latter already. Migrate what's left.

[1] https://github.com/testing-cabal/testtools/blob/2.8.0/NEWS#L33

Change-Id: I808be0f446d0b96ff4b10a6ceca8a61bbc80fa22
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
diff --git a/doc/source/write_tests.rst b/doc/source/write_tests.rst
index 3626a3f..0c604cc 100644
--- a/doc/source/write_tests.rst
+++ b/doc/source/write_tests.rst
@@ -83,7 +83,7 @@
           """
           super(TestExampleCase, cls).skip_checks()
           if not CONF.section.foo
-              cls.skip('A helpful message')
+              cls.skipTest('A helpful message')
 
       @classmethod
       def setup_credentials(cls):