contextual tests that the harness can ignore

 Module::Install::ExtraTests adds one Module::Install command:
 .
  extra_tests;
 .
 This declares that the test files found in the directory ./xt should be
 run only in certain instances:
 .
  ./xt/author - run when tests are being run in an author's working copy
  ./xt/smoke - run when the dist is being smoked (AUTOMATED_TESTING=1)
  ./xt/release - run during "make disttest"
 .
 These directories are recurisvely scanned for *.t files. If any
 directories or files exist in ./xt that are not recognized, the
 Makefile.PL will die.