trac#665: Upgrade to WebTest 2.0.18 and fix broken test `test_csrf_cookie_set`.

trac#665: Upgrade to WebTest 2.0.18 and fix broken test `test_csrf_cookie_set`.

Test was failing due to API change in WebTest around accessing cookies set in the test client. These are now in `test_app.cookies`.

While there are currently 48 other tests failing, I've verified that before and after this change that the same number fail. I've also checked that no other tests use the old API for accessing test client cookies.

I've pinned to version 2.0.18. My understanding is that it's generally a good idea to pin to a specific version where possible to avoid the "why has this suddenly broken" type bugs. This also seems appropriate since WebTest appears to be very stable.