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/kt2920.kt

@@ -0,0 +1,9 @@
+trait Tr<T> {
+    val v: T
+}
+
+class C : Tr<String> {
+    override val v = "OK"
+}
+
+fun box() = C().v