Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/vararg/kt796_797.kt

@@ -0,0 +1,8 @@
+fun <T> Array<T>?.get(i : Int?) = this!!.get(i!!)
+fun <T> array(vararg t : T) : Array<T> = t
+
+fun box() : String {
+    val a : Array<String>? = array<String>("Str", "Str2")
+    val i : Int? = 1
+    return if(a[i] == "Str2") "OK" else "fail"
+}