#2339. Add test for extension named type and a record type

[sgrekhov]

We have an interesting case here.
Without // SharedOptions=--enable-experiment=inline-class experimental flag extension type on (int i,) should be treated as an extension named type on record type (int,).
With // SharedOptions=--enable-experiment=inline-class it became an extension type with a excesive comma after the representation type.

So, we may have two tests here

extension type on 
//        ^^^^
// [analyzer] unspecified
// [cfe] unspecified
    (int i,) {}

And

// SharedOptions=--enable-experiment=inline-class
extension type on
    (int i,) {}
//        ^
// [analyzer] unspecified
// [cfe] unspecified

But the first one became an invalid when the extension types will stop to be an experiment but became the part of the language. So I added only the one tests expecting an error in the right place when the extension types will be shipped. If you believe that we have more tests here, then let me know

[eernstg]

LGTM, the addition of only this case is fine!