No Suggestion For IsAssignableFrom and IsNotAssignableFrom

[sahin52]

Hello,

When I use latest NUnit.Analyzers V4.2.0, I don't get any suggestion for
Assert.IsNotAssignableFrom<RedirectResult>(a);

But when I install NUnit V4.1.0, I am getting error for this line:
'Assert' does not contain a definition for 'IsNotAssignableFrom'

I think that should be fixed

[mikkelbu]

The work will be done as part of #790