Avoid creating arbitrary filenames for tmpdir on Testdir's constructor

Fixes #2751

testing/test_collection.py

@@ -569,7 +569,6 @@ class Test_getinitialnodes(object):
         col = testdir.getnode(config, x)
         assert isinstance(col, pytest.Module)
         assert col.name == 'x.py'
-        assert col.parent.name == testdir.tmpdir.basename
         assert col.parent.parent is None
         for col in col.listchain():
             assert col.config is config