Run tests in parallel

[heaths]

For CIs or core-triggered runs where we run every crate, we should parallelize test runs e.g., run our test script for each service directory in parallel (perhaps all after sdk/core just to make sure it passes first) e.g., using a -Parallel foreach loop in PowerShell instead of multiplexing to different agents that would otherwise consume too many machines.

AT least by default, we probably shouldn't run tests within a service directory in parallel because those often share test-resources.bicep and may not function well in parallel. We had a lot of those problems in the Azure/azure-sdk-for-net repo and many (most?) client libraries end up running live tests serially.