Skip ppm.t in Module::Build if running under PERL_CORE.

Skip ppm.t in Module::Build if running under PERL_CORE.

  From #p5p

  [17:54] <@Steffen> avar, is blead passing the M::B ppm.t test again?
  [17:54] <@Steffen> Asking because of the release.
  [17:55] * avar checks
  <snip>
  [18:19] <@avar> xdg: zomg plz fix it wtf bbq
  [18:19] <@avar> :)
  <snip>
  [19:07] <@xdg> avar, nopaste the failure and I'll look at it tonight.  Worst case, just rm ppm.t --
                 it's not critical and it's a finicky test.  (Passes for me.)
  [19:07] * xdg nap &
  [19:09] <@avar> willdo later when I get home, unless someone beats me to it *hint*