Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/traits/traitFuncCall.kt

@@ -0,0 +1,17 @@
+open class Foo() {
+    public fun k(): String = "K"
+}
+
+
+trait T: Foo {
+    public fun xyzzy(): String = o() + k()
+    public fun o(): String
+}
+
+class TImpl(): Foo(), T {
+    public override fun o(): String = "O"
+}
+
+fun box(): String {
+    return TImpl().xyzzy()
+}