Extend test

We verified that before the combination abstract/concrete for `x`
also led to AMEs. So we test it here explicitly, too.

Author: odersky

tests/run/t7475b.check

@@ -1,2 +1,4 @@
 2
 2
+2
+2

tests/run/t7475b.scala

@@ -1,11 +1,16 @@
 trait A { private val x = 1 }
 trait B { val x = 2 }
+trait C { val x: Int }
 trait C1 extends B with A { println(x) }
 trait C2 extends A with B { println(x) }
+trait C3 extends C with B { println(x) }
+trait C4 extends B with C { println(x) }
 
 object Test {
   def main(args: Array[String]): Unit = {
     new C1 { }
     new C2 { }
+    new C3 { }
+    new C4 { }
   }
 }