Enable venv tests, split out step-template, update CI

For microsoft#4806, microsoft#4034, others...

- Venv tests are now in the combined step-template for all CI
- PR validation build definition uses the step template
- CI validation build definition uses the step template
  - Runs all tests against all OS, Py 3.7 & 3.6 only
- Some flaky tests skipped (tracked in GH)
- Problems with test runs not finishing in Windows remains

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Author: d3r3kk