Better error messaging when failing to run tests

[drognanar]

Taken from https://developercommunity.visualstudio.com/t/VS-Test-Explorer-wont-run-tests/10713829

The issue the customer is describing is such that the testhost.exe was not found.
The error message is somewhat vague (where should the testhost.exe be located)? How do we fix this error?

By giving more information as to how the testhost.exe is located, it's possible that the customer would be able to resolve the issue on their own.

[nohwnd]

do you have some suggestion for better wording, because it depends on the test runtime provider, and it is all overall quite complicated how we run testhost. So it is not a problem that user would normally fix themselves.

[drognanar]

In this case the issue was the NuGet setup for package references was preventing testhost.exe from being copied to the bin folder.

Perhaps saying that for .NET core, it's usually expected for the testhost.exe to be present in the bin folder and for .NET core it's expected for it to be present in the VS/SDK folder might be helpful. Or saying to make sure that the Test.SDK reference is correctly restored.

It might be that this is an edge case scenario and we just let users lookup the VS feedback ticket for resolution.