classic-azure-ts-vm-provisioners is not covered by tests

[thomas11]

This example is not in the suite defined in misc/test/azure_test.go.

I suspect the reason is that it needs an SSH key pair as input, and there's no ready-made way in Azure to create one. We could, for instance,

• use ssh-keygen or similar tool locally on the GH runner, or
• create a key pair once and save it in this repo's GH secrets