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

testing/test_resultlog.py

@@ -83,19 +83,10 @@ class TestWithFunctionIntegration:
 
     def test_collection_report(self, testdir):
         ok = testdir.makepyfile(test_collection_ok="")
-        skip = testdir.makepyfile(test_collection_skip=
-            "import pytest ; pytest.skip('hello')")
         fail = testdir.makepyfile(test_collection_fail="XXX")
         lines = self.getresultlog(testdir, ok)
         assert not lines
 
-        lines = self.getresultlog(testdir, skip)
-        assert len(lines) == 2
-        assert lines[0].startswith("S ")
-        assert lines[0].endswith("test_collection_skip.py")
-        assert lines[1].startswith(" ")
-        assert lines[1].endswith("test_collection_skip.py:1: Skipped: hello")
-
         lines = self.getresultlog(testdir, fail)
         assert lines
         assert lines[0].startswith("F ")