Get tests to run during nightlies for KeyVaultProxy sample #15416
Description
The KeyVaultProxy sample added in PR #15123 has tests we should run as part of nightly tests, but doing so requires further workarounds to pull in more of our common code (e.g. Azure.Core.TestFramework) while not pushing too many repo policies (from a technical standpoint) on the sample, which can make it unnecessarily complex for a standalone sample, or even prevent it from running standalone. Keep mind these are intended to be downloaded from Samples Browser, so any "repo-isms" we add - like dealing with errors about including a PackageReference/@Version attribute - only complicate the sample.
/cc @weshaggard @jongio