Make running netfx assemblies via Mono opt-in on non-Windows platforms

[akoeplinger]

Description

Right now VSTest will run .NET Framework assemblies using Mono on Unix platforms. This was useful in the past but nowadays the more common usage is multi-targetting a test assembly for net462/net8 where the netfx assembly contains Windows-specific code.

This results in issues like dotnet/runtime#81395 where a test assembly tried to run on mono unexpectedly.

Steps to reproduce

$ dotnet new mstest

1. Edit the .csproj to contain <TargetFrameworks>net462;net8.0</TargetFrameworks> and <LangVersion>Latest</LangVersion>.

2. Run dotnet test

Expected behavior

Only runs the tests once, for the net8.0 assembly.

Actual behavior

Runs both the net8.0 and net462 assembly, the latter using Mono.

Diagnostic logs

Environment

macOS 14, dotnet 8

[nohwnd]

I see how that would be useful, it would have to be a new flag, to avoid breaking people who are relying on the current behavior. We could add it to dotnet test msbuild task, so you could put opt-out flag directly into your project (e.g. false). And as a new runconfiguration.