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_agreement.py

@@ -498,7 +498,7 @@ def test_confirm_agreement_page_persists(
     ('foo', 404),
 ))
 def test_show_public_agreement_page(
-    client,
+    client_request,
     mocker,
     endpoint,
     variant,
@@ -508,8 +508,9 @@ def test_show_public_agreement_page(
         'app.s3_client.s3_mou_client.get_s3_object',
         return_value=MockS3Object()
     )
-    response = client.get(url_for(
+    client_request.logout()
+    client_request.get_response(
         endpoint,
         variant=variant,
-    ))
-    assert response.status_code == expected_status
+        _expected_status=expected_status,
+    )