Refactor and update SDK tests to use lightweight VCR interface

We can't use VCR.py's "custom_patches" parameter because our
API._send_json_request() is RPC- rather than connection-oriented.  But
we can just instrument _send_json_request() directly, which is what we
do here.

We subclass vcr.cassette.Cassette to handle identical requests with
different responses, which was suggestd by @vickyliin as a workaround
for kevin1024/vcrpy#753.

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

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.

The tests are also now more patient with slow deletion operations.
I'd want to DRY up this logic if this pattern shows up in more places,
but it would require adding another level of indirection.  A @Retry
decorator isn't appropriate at the level of the test method, and a
context manager can't loop over its closure.

And re-apply the hack from 880635d by renaming test_logout to start with
a "z" so it runs last.

client/tests/sdk/data/setup_method.yml

@@ -0,0 +1,22 @@
+interactions:
+- request:
+    body: '{"username": "journalist", "passphrase": "correct horse battery staple
+      profanity oil chewy", "one_time_code": "112097"}'
+    headers: {}
+    method: POST
+    uri: api/v1/token
+  response: !!python/object:securedrop_client.sdk.JSONResponse
+    data:
+      expiration: '2024-02-29T23:13:01.444028Z'
+      journalist_first_name: null
+      journalist_last_name: null
+      journalist_uuid: ed64b0fa-0565-4b12-9c48-6cfe27a73fd9
+      token: Ilhha2ZHZWpVcFZQVmpKdWJOUzJYRG1YVUc4QXZWS0JiOGVNQXJycE5GUFUi.ZeDzXQ.10ODTj49yF7kxKbeMm9eaayR3Xo
+    headers:
+      connection: close
+      content-length: '290'
+      content-type: application/json
+      date: Thu, 29 Feb 2024 21:13:01 GMT
+      server: Werkzeug/2.2.3 Python/3.8.10
+    status: 200
+version: 1