Prevent PytestCollectionWarning for TestRequest, TestResponse

[vbraun]

Prevents mildly annoying warnings in pytest of the form:

PytestCollectionWarning: cannot collect test class 'TestResponse' because it has a __init__ constructor (from: test/test_foo.py)
    class TestResponse(webob.Response):

This has already been fixed for TestApp in 0339515, but TestRequest and TestResponse were missed. But pytest will try to collect all classes whose name starts with "Test". A quick grep confirms that these three are the only webtest classes starting with "Test".