Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated

compiler/testData/codegen/box/classes/diamondInheritance.kt

@@ -0,0 +1,24 @@
+// Changed when traits were introduced. May not make sense any more
+
+open class Base() {
+    public var v : Int = 0
+}
+
+open class Left() : Base() {}
+trait Right : Base {}
+
+class D() : Left(), Right
+
+fun vl(l : Left) : Int = l.v
+fun vr(r : Right) : Int = r.v
+
+fun box() : String {
+    val d = D()
+    d.v = 42
+
+    if (d.v != 42) return "Fail #1"
+    if (vl(d) != 42) return "Fail #2"
+    if (vr(d) != 42) return "Fail #3"
+
+    return "OK"
+}