test(auth): Mock oauth/token module in token exchange unit test

Because:

* token.verify() was not mocked in two token exchange tests
* Each jest.resetModules() creates a fresh OauthDB singleton that opens real MySQL/Redis connections; the second instantiation hangs past 10s in CI, causing an intermittent timeout
* Example failure: https://app.circleci.com/pipelines/github/mozilla/fxa/68720/workflow
s/fd17ddcd-5541-49fe-90c2-28cb3bdd86c7/jobs/661401

This commit:

* Adds jest.doMock('../../oauth/token', ...) to both token exchange tests so token.verify() never hits real infrastructure in unit tests

Author: vpomerleau

packages/fxa-auth-server/lib/routes/oauth/token.spec.ts

@@ -745,6 +745,9 @@ describe('/oauth/token POST', () => {
       jest.doMock('../utils/oauth', () => ({
         newTokenNotification: newTokenNotificationStub,
       }));
+      jest.doMock('../../oauth/token', () => ({
+        verify: sinon.stub().resolves({ user: UID }),
+      }));
       const routes = require('./token')({
         ...tokenRoutesArgMocks,
         db: {
@@ -828,6 +831,9 @@ describe('/oauth/token POST', () => {
       jest.doMock('../utils/oauth', () => ({
         newTokenNotification: newTokenNotificationStub,
       }));
+      jest.doMock('../../oauth/token', () => ({
+        verify: sinon.stub().resolves({ user: UID }),
+      }));
       const routes = require('./token')({
         ...tokenRoutesArgMocks,
         db: {