Warn when MSTest.TestFramework and MSTest.TestAdapter versions are not aligned

[Youssef1313]

It's not uncommon for projects to end up with MSTest.TestFramework version being not aligned with MSTest.TestAdapter, which is usually not a good idea. We can consider issuing a warning in this case.

Open questions

• Is the benefit worth it?
• Any potential false positives?
• Should we issue a warning only if both packages are explicitly referenced (i.e, direct dependency)? Or should we still issue a warning if TestFramework is transitive while TestAdapter is direct?
• How should this be implemented?
  • Analyzer? Not going to be widely useful as analyzers are opt-in and many projects don't use it.
  • MSBuild target in TestFramework or TestAdapter package?
  • The new MSBuild Check feature (if it's capable of doing so)?

Example: https://github.com/microsoft/accessibility-insights-windows/blob/54d160cb743a6ab30d3b763b655c7b480af4c85d/src/UITests/UITests.csproj#L17-L18

[Evangelink]

I'd vote for MSBuild check