tests: add TestMasqueradePrivilege auth-flow tests HEAD main

Test that POST /auth/user/masquerade/ enforces the
system:user:masquerade privilege.  Both admin and basic users can
obtain a token with masquerade scope (the test client supports it),
but gn-auth's can_masquerade decorator checks the user's roles:

  - Basic user → ForbiddenAccess → 403
  - Admin user → can masquerade as basic user → 200

Adds three session-scoped fixtures to the test module: admin_masquerade_token,
basic_masquerade_token, and basic_user_id.

0 files changed, 0 insertions, 0 deletions