Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/when/nullableWhen.kt

@@ -0,0 +1,13 @@
+// KT-2148
+
+
+fun f(p: Int?): Int {
+    return when(p) {
+        null -> 3
+        else -> p!!
+    }
+}
+
+fun box(): String {
+    return if (f(null) == 3) "OK" else "fail"
+}