Support VSTest parallel test execution setting

[dungpa]

This PR enables the use of /Parallel setting in vstest.console.exe.

I haven't added /Parallel to the legacy counterpart at https://github.com/fsharp/FAKE/blob/1f8de0538209376f73e964f71957ddb6dabaa45c/src/legacy/FakeLib/UnitTest/VSTest.fs#L34. Please let me know if the setting is also required there.

See these references for more details about the setting:

• https://docs.microsoft.com/en-us/visualstudio/test/vstest-console-options?view=vs-2017
• https://devblogs.microsoft.com/devops/parallel-test-execution/

TODO

Feel free to open the PR and ask for help

• New (API-)documentation for new features exist (Note: API-docs are enough, additional docs are in help/markdown)

• unit or integration test exists (or short reasoning why it doesn't make sense)

  Note: Consider using the CreateProcess API which can be tested more easily, see https://github.com/fsharp/FAKE/pull/2131/files#diff-4fb4a77e110fbbe8210205dfe022389b for an example (the changes in the DotNet.Testing.NUnit module)

• boy scout rule: "leave the code behind in a better state than you found it" (fix warnings, obsolete members or code-style in the places you worked in)

• Fake 5 API guideline is honored

[matthid]

Thanks for looking into this! I noticed this module doesn't use properly escaped APIs and has no tests. Maybe you could clean it up?

[dungpa]

Could you give some pointers where we use properly escaped APIs and test samples that I can follow? (Sorry, I haven't looked at the code base for a long time).

[matthid]

Sure, we have https://fake.build/core-process.html and in particular the Arguments API

In the end it should look similar to the StringBuilder which is currently used, but we can have a internal "helper" function returning the Arguments object which we can write tests against. Just look at the existing tests.

[dungpa]

@matthid I added tests as requested.

[matthid]

Ah I didn't realize this only writes an argument file. Anyway, thanks for adding the test!