How are MTP test projects discovered when dotnet test is run against a solution containing them?

[IanKemp]

When my org started containerising our code, we discovered our test projects weren't being discovered by dotnet test <solution> (where <solution> refers to a sln(x) containing at least one test project referencing Microsoft.Net.Test.Sdk) when tests were executed in Docker builds. Subsequent investigation revealed this is because projects referencing that package, implicitly get IsTestProject=true set via a .props file shipped with it - and in our Dockerfile's test run layer we'd omitted copying the NuGet packages, which meant that file didn't exist. (sad trombone noise)

We're now considering a move to MTP. Given that it doesn't appear to require Microsoft.Net.Test.Sdk so presumably doesn't care about the presence or absence of IsTestProject, we'd thus like to understand how dotnet test <solution> discovers MTP projects - so that we can ensure our Dockerfiles don't do anything that would prevent this discovery from working as we expect.