Libraries referencing a test project will fail when running dotnet test

[Youssef1313]

Describe the bug

When running tests with VSTest, dotnet test is not going to fail if a project doesn't have IsTestProject set to true. This is because of the target having a condition on IsTestProject:

https://github.com/microsoft/vstest/blob/4b7642501171db13c4a942fbf5dbe443dffd0fd7/src/Microsoft.TestPlatform.Build/Microsoft.TestPlatform.targets#L44

For MTP, if the library is setting TestingPlatformDotnetTestSupport (which is common because we want to advise users to set it in Directory.Build.props), then we will attempt to run the library as a project.

We should consider adding IsTestProject condition to InvokeTestingPlatform target.

Alternatively, we could add <TestingPlatformDotnetTestSupport Condition="'$(IsTestProject)' != 'true'">false</TestingPlatformDotnetTestSupport> after this line:

testfx/src/Platform/Microsoft.Testing.Platform.MSBuild/buildMultiTargeting/Microsoft.Testing.Platform.MSBuild.targets

Line 237 in 9725b59

<TestingPlatformDotnetTestSupport Condition=" '$(TestingPlatformDotnetTestSupport)' == '' ">False</TestingPlatformDotnetTestSupport>

Steps To Reproduce

Run dotnet test on the attached solution.

TestProject32.zip

Note: It works with VSTest.

Note 2: It's not a good practice that a library is referencing a test project and is usually a sign of a bad design of some sort.

Expected behavior

Actual behavior

Additional context

[Youssef1313]

@Evangelink I'm classifying as a bug as I think TestingPlatformDotnetTestSupport shouldn't have an impact on non-test projects

[Youssef1313]

As we discussed offline, IsTestProject is not the right thing to do as it's VSTest property.