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.