Add a test case that makes use of a virtual-security-domain with MP-JWT

WFLY-16793 introduced the virtual-security-domain resource. The tests added so far have focused on testing this new resource with OIDC but MP-JWT also makes use of virtual security domains. This task is to add a test case that involves MP-JWT with a corresponding virtual-security-domain defined.

The documentation for virtual-security-domains should also be updated to mention the auth-method attribute.