Replace uses of client.get and client.post

We have a `client_request` fixture which does a bunch of useful stuff
like:
- checking the status code of the response
- returning a `BeautifulSoup` object

Lots of our tests still use an older fixture called `client`. This is
not as good because it:
- returns a raw `Response` object
- doesn’t do the additional checks
- means our tests contain a lot of repetetive boilerplate like `page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')`

This commit converts all the tests which had a `client.get(…)` or
`client.post(…)` statement to use their equivalents on `client_request`
instead.

Subsequent commits will remove uses of `client` in other tests, but
doing it this way means the work can be broken up into more manageable
chunks.

tests/app/main/views/test_platform_admin.py

@@ -25,12 +25,14 @@ from tests.conftest import SERVICE_ONE_ID, SERVICE_TWO_ID, normalize_spaces
     'main.trial_services',
 ])
 def test_should_redirect_if_not_logged_in(
-    client,
+    client_request,
     endpoint
 ):
-    response = client.get(url_for(endpoint))
-    assert response.status_code == 302
-    assert response.location == url_for('main.sign_in', next=url_for(endpoint), _external=True)
+    client_request.logout()
+    client_request.get(
+        endpoint,
+        _expected_redirect=url_for('main.sign_in', next=url_for(endpoint), _external=True),
+    )
 
 
 @pytest.mark.parametrize('endpoint', [