ModelDirectiveTest: Fix incorrect test assertions

Both of the design-time tests fail after being updated to use code documents from the project engine. The reason is that the test was initially buggy. Previously, the design-time tests created a RazorProjectEngine with options configured for design-time. However, the RazorCodeDocument was created without those options. So, DocumentIntermediateNode.Options.DesignTime wouldn't be set, and ModelDirective.Pass.ExecuteCore(...) would follow a runtime code path. Now that the RazorCodeDocument is correctly configured, it takes the correct design-time code path and the assertions need to be updated.

IOW, the test was wrong and now it's correct.

Author: DustinCampbell

src/Compiler/Microsoft.AspNetCore.Mvc.Razor.Extensions/test/ModelDirectiveTest.cs

@@ -227,7 +227,7 @@ @inherits BaseType<TModel>
         var baseType = @class.BaseType;
 
         Assert.Equal("BaseType", baseType.BaseType.Content);
-        Assert.NotNull(baseType.BaseType.Source);
+        Assert.Null(baseType.BaseType.Source);
 
         Assert.NotNull(baseType.ModelType);
         Assert.Equal("dynamic", baseType.ModelType.Content);
@@ -265,7 +265,7 @@ @model SomeType
         var baseType = @class.BaseType;
 
         Assert.Equal("BaseType", baseType.BaseType.Content);
-        Assert.NotNull(baseType.BaseType.Source);
+        Assert.Null(baseType.BaseType.Source);
 
         Assert.NotNull(baseType.ModelType);
         Assert.Equal("SomeType", baseType.ModelType.Content);