Engine extensions do not work with our dotnet tool

[CharliePoole]

Because the dotnet tool is a standalone executable, extensions are difficult to locate because they are installed about eight directory levels above the runner and engine. This requires a more agressive approach in searching for the extensions.

Fixing this depends on #1504

[CharliePoole]

@OsirisTerje @veleek This info may be helpful...

The following extensions have a .NET Standard or .NET Core target and should work...

• NUnit V2 Result Writer
• NUnit Project Loader (note 1)

The following extensions do not yet have a .NET Standard or .NET Core target, and therefore cannot be used at this time...

• TeamCity Event Listener (note 2)
• Visual Studio Project Loader (note 1)
• NUnit V2 Driver (note 3)

NOTES:

1. The project loaders present a peculiar issue because the projects may not contain incompatible asemblies. It's not clear what we should do if that occurs. Thoughts?
2. TeamCity Event Listener version 1.0.9 has a .NET Standard build, but we are still using 1.0.7 because that build was only published on nuget.org and not on our other distribution points. We could test using 1.0.9 if desired. @NikolayPianikov What do you think?
3. This will never work, since NUnit V2 tests can only run under the .NET Framework.

[CharliePoole]

I believe this should work with the current code provided the extension is compatible - i.e. provides a .NET Standard or Core version. I'll close the issue after the following steps are done

• Add a test showing that the V2 Result Writer works.
• Add tests showing that each of the other extensions fails in the expected way.

Further issues should then be raised with the appropriate extension.