Correct mockAdminHandler behavior in MockClient

[ThatOneBro]

mockAdminHandler strays far from the real implementation of the admin routes and is currently breaking at least a dozen tests in @medplum/app and maybe some in other packages as well.

Realistically we should probably enumerate at least different classes of routes in that handler and if there is no match by the end, we should throw. This should greatly reduce the amount of bugs that this part of MockClient could introduce