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/kt870.kt

@@ -0,0 +1,5 @@
+fun box() = when {
+        1 > 2 -> "false"
+        1 >= 1 -> "OK"
+        else -> "else"
+    }