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/matchNotNullAgainstNullable.kt

@@ -0,0 +1,7 @@
+fun foo(i: Int, j: Int?): String =
+    when (i) {
+        j -> "OK"
+        else -> "Fail"
+    }
+
+fun box(): String = foo(0, 0)