Misleading error message when test project is missing test runner

[TheRealPiotrP]

Moved from https://github.com/dotnet/cli/issues/5135 on behalf of @bleroy

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Steps to reproduce

Create a new console app, add the xunit package, write a few tests, try to dotnet restore and dotnet test.

Expected behavior

An error message saying to add a test runner, ideally suggesting the right one based on what testing framework is referenced if we have a way to recognize that at this point.

Actual behavior

An error message suggesting to add "microsoft.testplatform.testhost". The right packages to add to make it work really are Microsoft.NET.Test.Sdk and xunit.runner.visualstudio.

Environment data

Reproduces on Preview 3 and Preview 4.

[Faizan2304]

For one more scenario this message should be improved:

Scenario
- Create a unit test project targeting net46
- Try to run it with "dotnet vstest path-of-unit-test-net46-dll"

It should give message to check framework as well because when we run test from "dotnet vstest"
the default framework is netcoreapp1.0.