Test Discovery can influence/break design time builds

[TheAngryByrd]

Describe the bug

ionide/FsAutoComplete#1364 (comment)

Steps to reproduce

0. do a git clean -xdf; dotnet restore
1. Open a project that has tests
2. See the error presented in the FSAC issue

Link to sample reproduction

FsToolkit.ErrorHandling

Expected behaviour

We should probably do this test discovery/building in a separate folder so we don't collide with design time builds.

Screenshots

Machine infos

• OS: Windows/Mac/Linux
• .NET SDK version: [e.g. 5.0.100]
• Ionide version: [e.g 5.0.1, don't use descriptions like latest ]

Additional context

Add any other context about the problem here.

[baronfel]

Also from ionide/FsAutoComplete#1364 (comment)

I think the ultimate fix is to have our test discovery run with different BaseIntermediateOutputPath and BaseOutputPath.

Yes, this is perfectly fine. We may also want test discovery and any other per-project operations to use BuildProjectReferences=false to skip building referenced projects to prevent unnecessary re-builds.

[farlee2121]

We could maybe side-step the issue by moving to VSTest, which I had been looking into for other reasons.

But, that might get invalidated with the new testing platform.
I'd guess the new test platform needs to be built externally, since the test project itself hosts the test runner. Though, there is much I don't understand yet.

[farlee2121]

I got around to working samples of VSTest and Microsoft Testing Platform.
Neither is going to solve build challenges for us.

I think a change like this one makes a lot of sense.