Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/strings/rawStrings.kt

@@ -0,0 +1,6 @@
+fun box() : String {
+  val s = """ foo \n bar """
+  if (s != " foo \\n bar ") return "Fail: '$s'"
+
+  return "OK"
+}