[Release/3.0] Set IsShipping=false for all TestingUtils projects

[wtgodbe]

Without this, the ref project(s) in this dir get the default value of IsShipping=true from Arcade: https://github.com/dotnet/arcade/blob/0c9b0442f2ce607a90615a7bfad4ead069150bc6/src/Microsoft.DotNet.Arcade.Sdk/tools/Version.BeforeCommonTargets.targets#L18. This means that other projects which compile against ref projects in this folder (i.e., Microsoft.Extensions.Logging.Testing compiles against Microsoft.AspNetCore.Testing) will get non-suffixed versions for their dependencies. When we ship stable, Microsoft.Extensions.Logging.Testing lists a dependency on a stable version of Microsoft.AspNetCore.Testing, which does not exist.

We should think of a better way to resolve this issue - for example, AspNetCore calculates a global default for IsShipping, which would prevent this type of issue: https://github.com/aspnet/AspNetCore/blob/20fc1adf2ada01a4a1fc7934706d2b641acbb9f4/Directory.Build.props#L23

It's also worth noting that Microsoft.AspNetCore.Testing is unique in that it is a non-shipping project that has a ref assembly. We should understand why that ref assembly is needed, and decide if it can be removed (removing it caused some errors that seemed non-trivial to fix). The ref assembly was added in this PR: #2483

CC @dougbu @JunTaoLuo @mmitche

[wtgodbe]

I'll be back at my desk in around ~45 minutes, let's wait for @dougbu's signoff before merging

[dougbu]

This is fine and won't need to be reverted later.

Please file a follow-up issue to get to the bottom of why we added the ref/ project for a non-shipping package. That guy should be removed ASAP.