Expand tests to account for audit access policy

[alpeb]

Followup to #12846, branched off alpeb/policy-audit-impl

This fixes the policy controller unit and integration tests by accounting for the new Audit default policy and the new accessPolicy field in Server.

New integration tests added:

• e2e_audit.rs exercising first the audit policy in Server, and then at the namespace level
• in admit_server.rs a new test checks invalid accessPolicy values are rejected.
• in inbound_api.rs server_with_audit_policy verifies the synthesized audit authorization is returned for a Server with accessPolicy=audit

Please check linkerd/website#1805 for how this is supposed to work from the user's perspective.