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/kt1976.kt

@@ -0,0 +1,8 @@
+class A {
+    public val f : ()->String = {"OK"}
+}
+
+fun box(): String {
+    val a = A()
+    return a.f() // does not work: (in runtime) ClassCastException: A cannot be cast to jet.Function0
+}