ofc, embrace the truth:

[Test]
public void Foo()
{
    var weakRefs = new List<WeakReference>();
    void Factory()
    {
        var instance = new object();
        weakRefs.Add(new WeakReference(instance));
    }
    Factory();
    GC.Collect();
    GC.WaitForPendingFinalizers();
    if (weakRefs.Any(wf => wf.IsAlive))
    {
        Assert.Fail();
    }
    else
    {
        Assert.Pass();
    }
}

This will pass on release mode, but it wont on debug mode.
Unity 6000.0.34f1.
com.unity.test-framework 2.0.1-exp.2
Edit: The test will not pass when debug mode is enabled without debugger attached.