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/kt1978.kt

@@ -0,0 +1,10 @@
+fun aa(vararg a : String): String = a[0]
+
+fun box(): String {
+    var result: String = ""
+    var i = 1
+    while (3 > i++) {
+        result = aa(if (true) "OK" else "fail")
+    }
+    return result
+}