Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/bridges/kt1939.kt

@@ -0,0 +1,12 @@
+abstract class Foo<T> {
+    fun hello(id: T) = "Hi $id"
+}
+
+trait Tr {
+    fun hello(s : String)
+}
+
+class Bar: Foo<String>(), Tr {
+}
+
+fun box(): String = if (Bar().hello("Reg") == "Hi Reg") "OK" else "Fail"