Update the authentication flow integration test to verify that the
pomerium_identity_profile cookies are not present for the stateful
authentication flow.
Commit 08c186a contains a bug in the integration configuration template,
preventing the multi-stateless cluster from actually setting the
DEBUG_FORCE_AUTHENTICATE_FLOW environment variable. As a result this
cluster was not exercising the stateless authentication flow.
Fix the template so that this variable is applied as intended.
Add an integration test case to verify that the intended authentication
flow is in use: for the stateful flow, different routes should share the
same underlying session, but for the stateless flow, different routes
should receive different sessions.
