Cleanup a few compiler flags in test/files/

 * String interpolation isn't Xexperimental anymore

   A few useless Xexperimental flags in tests were left behind by 6917cca,
   after string interpolation was made non-experimental in 983f414.

 * Things added under -Xfuture in 2.10 are very much Xpresent now, the
   flag isn't needed anymore.