Run CI tests on VSTS for PRs from core contributors of extension

[DonJayamanne]

The core contributors of the extension have to wait for the tests to pass before we can merge the tests (sometimes the tests take forever, sometimes they need to be re-run - due to flaky tests).
All of this slows down the time taken to merge a PR.

Solution:

• Run tests on VSTS (all platforms)
• Once tests pass, merge the PR
• If tests fail on Master (AppVeyor or Travis), then
  • Restart tests
  • Worst case, identify the problem (identify why tests pass on VSTS and not on AppVeyor/Travis)
• No changes to external contributors

@brettcannon @MikhailArkhipov
Thoughts

[brettcannon]

We should set up VSTS to get the 3-platform coverage anyway.

[DonJayamanne]

Closing as we've commenced work on this, and tracked in a separate issue.