update testing.md - fixes related to RequestsClient (#5959)

* Include import for RequestsClient in the docs.
* Use fully qualified URLs for `RequestsClient` in the docs.

Author: cgl

docs/api-guide/testing.md

@@ -201,6 +201,8 @@ live environment. (See "Live tests" below.)
 This exposes exactly the same interface as if you were using a requests session
 directly.
 
+    from rest_framework.test import RequestsClient
+    
     client = RequestsClient()
     response = client.get('http://testserver/users/')
     assert response.status_code == 200
@@ -237,12 +239,12 @@ For example...
     client = RequestsClient()
 
     # Obtain a CSRF token.
-    response = client.get('/homepage/')
+    response = client.get('http://testserver/homepage/')
     assert response.status_code == 200
     csrftoken = response.cookies['csrftoken']
 
     # Interact with the API.
-    response = client.post('/organisations/', json={
+    response = client.post('http://testserver/organisations/', json={
         'name': 'MegaCorp',
         'status': 'active'
     }, headers={'X-CSRFToken': csrftoken})