When switching to FrameworkReferences, discovered a problem with some tests for AzureInstanceMetadada (see full write up in #2357).

This PR superceedes #2358
This is a less invasive change that doesn't require changes to the call stack.

Changes

• Replace local inproc server with HttpClient and mock HttpMessageHandler.
• Remove test AzureImsResponseTooLargeStopsCollection. HttpClient correctly handles and parses the larger response.
• Replace test AzureImsResponseTimesOut with AzureImsResponseHandlesException. Could not force a timeout in a test context. HttpClient throws an exception when timeout. I changed this test to throw an exception and verify that this is handled correctly.

Checklist

• I ran Unit Tests locally.
• CHANGELOG.md updated with one line description of the fix, and a link to the original issue if available.

For significant contributions please make sure you have completed the following items:

• Design discussion issue #
• Changes in public surface reviewed

The PR will trigger build, unit tests, and functional tests automatically. Please follow these instructions to build and test locally.

Notes for authors:

• FxCop and other analyzers will fail the build. To see these errors yourself, compile localy using the Release configuration.

Notes for reviewers:

• We support comment build triggers
  • /AzurePipelines run will queue all builds
  • /AzurePipelines run <pipeline-name> will queue a specific build