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/kt454.kt

@@ -0,0 +1,5 @@
+fun box(): String {
+    var s1 = (@l1 "s")
+    val s2 = (@l2 if (@l3 true) s1 else null)
+    return if (s2 == "s") "OK" else "fail"
+}