Fix `test_requires_ext_decorator_with_all_ext_enabled`

This test raised a wrong `TestCase.skipException`. The problem was that
the config flag `compute-feature-enabled.api_extensions` is a
`cfg.ListOpt` and as such, it expects a `list`, not a `str`.

Change-Id: I4937d024c7349f9e76d5f632852448ab5ce56576
diff --git a/tempest/tests/test_decorators.py b/tempest/tests/test_decorators.py
index 98b045a..da5e2d7 100644
--- a/tempest/tests/test_decorators.py
+++ b/tempest/tests/test_decorators.py
@@ -201,7 +201,13 @@
         if expected_to_skip:
             self.assertRaises(testtools.TestCase.skipException, t.test_bar)
         else:
-            self.assertEqual(t.test_bar(), 0)
+            try:
+                self.assertEqual(t.test_bar(), 0)
+            except testtools.TestCase.skipException:
+                # We caught a skipException but we didn't expect to skip
+                # this test so raise a hard test failure instead.
+                raise testtools.TestCase.failureException(
+                    "Not supposed to skip")
 
     def test_requires_ext_decorator(self):
         self._test_requires_ext_helper(expected_to_skip=False,
@@ -213,7 +219,7 @@
                                        service='compute')
 
     def test_requires_ext_decorator_with_all_ext_enabled(self):
-        cfg.CONF.set_default('api_extensions', 'all',
+        cfg.CONF.set_default('api_extensions', ['all'],
                              group='compute-feature-enabled')
         self._test_requires_ext_helper(expected_to_skip=False,
                                        extension='random_ext',