Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/controlStructures/kt1742.kt

@@ -0,0 +1,7 @@
+fun box(): String {
+    val x = 2
+    return when(x) {
+        in (1..3) -> "OK"
+        else -> "fail"
+    }
+}