Fix dotnet-cli errors from `dotnet test`

[mgravell]

Running dotnet test currently gives failure messages:

Starting test execution, please wait...
Could not find testhost.dll for source 'C:\code\System.Ben\tests\bin\Debug\netstandard1.0\System.Ben.Tests.dll'. Make sure test project has a nuget reference of package "microsoft.testplatform.testhost".

This is clearly sub-optimal and needs fixing; the perfect fix is sadly hampered by a bug in xunit/VSTest.Console, which gives the following incorrect result:

No test is available in C:\code\System.Ben\tests\bin\Debug\netcoreapp1.1\System.Ben.Tests.dll. Make sure that installed test discoverers & executors, platform & framework version settings are appropriate and try again.

I have therefore used a workaround with:

Total tests: 1. Passed: 0. Failed: 0. Skipped: 1.
Test Run Successful.
Test execution time: 1.5873 Seconds