Sends Perl::Tidy errors during testing to a string instead of fs (GH#9)

• By default Perl::Tidy writes a perltidy.ERR file when it encounters
  errors. Perl::Tidy::Sweetened tests remove the error file then check
  to see if it is created after each test. This leads to a race
  condition with running in parallel (ie, prove -j 9). This commit makes
  Perl::Tidy write errors to a string buffer instead. Thanks @shlomif
  for the bug report (GH#9).
  [Only changes tests. No need to upgrade.]