Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/extensionFunctions/kt865.kt

@@ -0,0 +1,19 @@
+import java.util.*
+
+class Template() {
+    val collected = ArrayList<String>()
+
+    fun String.plus() {
+       collected.add(this@plus)
+    }
+
+    fun test() {
+        + "239"
+    }
+}
+
+fun box() : String {
+    val u = Template()
+    u.test()
+    return if(u.collected.size() == 1 && u.collected.get(0) == "239") "OK" else "fail"
+}