Raise CollectError if pytest.skip() is called during collection

pytest.skip() must not be used at module level because it can easily be
misunderstood and used as a decorator instead of pytest.mark.skip, causing the
whole module to be skipped instead of just the test being decorated.

This is unexpected for users used to the @unittest.skip decorator and therefore
it is best to bail out with a clean error when it happens.

The pytest equivalent of @unittest.skip is @pytest.mark.skip .

Adapt existing tests that were actually relying on this behaviour and add a
test that explicitly test that collection fails.

fix #607

_pytest/python.py

@@ -656,6 +656,12 @@ class Module(pytest.File, PyCollector):
                 "Make sure your test modules/packages have valid Python names."
                 % (self.fspath, exc or exc_class)
             )
+        except _pytest.runner.Skipped:
+            raise self.CollectError(
+                "Using @pytest.skip outside a test (e.g. as a test function "
+                "decorator) is not allowed. Use @pytest.mark.skip or "
+                "@pytest.mark.skipif instead."
+            )
         #print "imported test module", mod
         self.config.pluginmanager.consider_module(mod)
         return mod