Use TestApp.reset() instead of accessing the cookiejar directly.

It should make the test more compatible with older WebTest versions

test/test_oauth.py

@@ -344,7 +344,7 @@ class OAuthTC(CubicWebTestTC):
         self.login()
         self.assertEqual('zeuser', self.webapp.cookies['__data_session'][:6])
 
-        self.webapp.cookiejar.clear()
+        self.webapp.reset()
 
         self.login(oauthtoken="SOMETOKEN", autologin=True)
         self.assertEqual('zeuser', self.webapp.cookies['__data_session'][:6])