Update `SimulateTokenRequest` test helper to enable returning mocked responses when GetToken is called on an underlying credential type that doesn't match the input credential

[ahsonkhan]

This case shows up when a credential type like AzurePipelinesCredential or WorkloadIdentityCredential defers the GetToken call to another credential (ClientAssertionCredential) rather than doing the work directly.

See #5802 (comment)

[ahsonkhan]

Fixed in #5802 (comment)