test: merge TestAPIProxy into TestAPI

Now that the SDK‒proxy connection is itself instrumented, there's only
one path to test, with no special error-handling logic required.

I considered merging TestAPI and TestShared as well, now that (without
TestAPIProxy) TestAPI is the only subclass of TestShared.  But
reorganizing the alphabetized helpers in TestShared versus the
strictly-sequenced TestAPI methods can wait.

client/tests/sdk/data/setup_method.yaml

@@ -0,0 +1,22 @@
+interactions:
+- request:
+    body: '{"username": "journalist", "passphrase": "correct horse battery staple
+      profanity oil chewy", "one_time_code": "253705"}'
+    headers: {}
+    method: POST
+    uri: api/v1/token
+  response: !!python/object:securedrop_client.sdk.JSONResponse
+    data:
+      expiration: '2024-02-02T04:32:07.217091Z'
+      journalist_first_name: null
+      journalist_last_name: null
+      journalist_uuid: 8248d25d-f6f6-4ad3-a6d4-a287641d533c
+      token: IldNSTRhZFdmaDRQb0I3MzNYNmR0NkR4OFFrNDhsZEl0U05teVFyZEs4UTQi.ZbxUJw.JbZXOTTX32PYHCLs5X8sr7s10KI
+    headers:
+      connection: close
+      content-length: '265'
+      content-type: application/json
+      date: Fri, 02 Feb 2024 02:32:07 GMT
+      server: Werkzeug/2.2.3 Python/3.8.10
+    status: 200
+version: 1