Workload install/uninstall/list tests have issues

[mmitche]

The workload install/uninstall and list tests (VerifyWorkloadCmd and maybe some others) have some issues:

• They will pick up global state, like from a VS installation. So even if you uninstall wasm-tools, the test would fail because VS still had it installed.
• Tests running in parallel may stamp on each other. E.g. wasm-tools test A might uninstall the workload will wasm-tools test B is running.