Test-Setup.ps1 in core does not work

A [recent change to azure-amqp](https://github.com/Azure/azure-amqp/blob/master/global.json) has broken the `Test-Setup.ps1` script for the core pipeline.

Suggested approaches:
1. Try a different verison of .NET OR
2. Pin the ref used to obtain azure-amqp ([done in Rust SDK](https://github.com/Azure/azure-sdk-for-rust/blob/main/sdk/core/azure_core_amqp/Test-Setup.ps1#L39)) so that its build is predictable
3. Maybe a combination of 1 and 2

Discussed this with @LarryOsterman last week. I think the change should be straightforward.