Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/closures/kt2151.kt

@@ -0,0 +1,11 @@
+fun foo(): String {
+    return if (true) {
+        var x = "OK"
+        fun foo() { x += "fail" }
+        x
+    } else "fail"
+}
+
+fun box(): String {
+    return foo()
+}