Build process maintenance 4th

[9rnsr]

Related: #1033

• Migrate csproj for net472 to SDK style.
  Add TargetFramework net427 to *.Core.csproj files, then remove old style csprojs.

• Fix troublesome manual .runsettings file specification for testing.
  Add default runsettings file and use it by adding <RunSettingsFilePath> setting in Directory.Build.props.
  After the SDK style csproj migrations, any tests built for all TargetFrameworks will have same relative filepaths to the required test data. Therefore we can have one test.runsettings file at the root of repository.

[9rnsr]

I fixed the failure of AppVeyor build (Internal server error).
We cannot enable generation of XML documentation files in Debug build, because it will report many many CS1591 warnings.

[tonyqus]

I don't think merge Core csproj and non-Core csproj is a good idea. I remember there are some test cases behaving differently between Core and non-Core mode.

[9rnsr]

Today, tests in Core csprojs only targets net6.0, but we can easily add one more target framework net472.

[9rnsr]

If there is no reasons other than the test behavior, I prefer to use SDK style csprojs for maintainability.

[tonyqus]

I'll evaluate if it's possible to merge Core csproj and non-Core csproj. The situation is different from before. NPOI doesn't need to support .NET framework 2.x/3.x anymore. The difference may have became very tiny.

[9rnsr]

OK, I added net472 for three test projects. After that, test explorer will display two target frameworks for each test projects. We can do testing in specific target framework, or run them all at once.

[tonyqus]

LGTM

[9rnsr]

Thanks!

[9rnsr]

This pull request introduced a regression. Look at the fix in #1083.