Add more `ExpressionHelper` tests

[dougbu]

Following up on PR #5782 and @pranavkm's question, should add tests covering other lambda expressions that might start with "model" e.g.

var model = "Some string";
var text = ExpressionHelper.GetExpressionText((Expression<Func<TestModel, string>>)(m => model));

or

var text = ExpressionHelper.GetExpressionText((Expression<Func<TestModel, int>>)(m => MyApp.Model.Constants.CopyrightYear));

or

var text = ExpressionHelper.GetExpressionText(Expression<Func<TestModel, string>>)(m => StaticClass.Model.Name));

Not saying we need to fix problems if any of the above confuse ExpressionHelper. But, we should know how they behave and perhaps warn people (in docs) if some choices behave strangely.

[dougbu]

2cabd58