Change CLI test to use a dedicated, locked down user

[thomas11]

This is a follow-up to #3625. The test introduced in that issue works, but uses a real user's token which is not optimal. Ideally, we would have a dedicated artificial user for testing, whose permissions would be locked down.

The main challenge is that Azure requires 2FA.

[EronWright]

Something new on this front, since https://github.com/pulumi/pulumi-azure-native/pull/4242 was merged it might now be possible to login to the Azure CLI using a service principal as opposed to a user account. Previously, the system checked that the login was that of a user, now it doesn't. Since our goal here is to test the CLI auth mode, it shouldn't matter the type of account.