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/kt2702.kt

@@ -0,0 +1,14 @@
+open class A<R> {
+    open fun foo(r: R): R {return r}
+}
+
+open class B : A<String>() {
+}
+
+open class C : B() {
+    override fun foo(r: String): String {
+        return super.foo(r) + "K"
+    }
+}
+
+fun box() = C().foo("O")