Test project templates

[codito]

(This issue is a discussion follow up to the initial PR for test templates).

Following are the requirements for the test template:

1. Emit runtimeconfig.*.json on build. This is required for the dotnet vstest and dotnet test3 commands to setup the --additionalProbingPath for dotnet exec so that nuget references are resolved.
2. Allow P2P reference to all user projects (both class libraries and console apps).
3. Allow publish of the test project. This will allow user to run tests in a machine which doesn't have a source code enlistment e.g. a CDP flow, where tests are run in a Release Definition (usually acceptance tests on a Unix or Windows machine with her product installed)
4. Test project shouldn't include a program entry point. The entry point is always the testhost which comes from the test platform (see Microsoft.TestPlatform.TestHost nuget package).

If we choose a Console App based project template, (4) may need work.
If we choose a Class Library based project template, (4) is met, but 1,3 may need work.

Notes
(1) was supported with a special check in older project model (see Microsoft.Dotnet.ProjectModel).
(4) was supported with emitEntryPoint: false in project.json.

[codito]

@eerhardt @livarcocc @piotrpMSFT @srivatsn thoughts?

[livarcocc]

@codito We discussed this today and we believe that the best approach is to go the console app approach.

[srivatsn]

Since we are going with the console app approach, we should generate the entry point through the targets I think and not require the templates to have a dummy main method.

[srivatsn]

This is done.