skip test_readonly on windows

skip test_readonly on windows

       drop vs readonly repo failed (transcript follows)
      drop foo
      failed
      git-annex: .git\annex\objects\6cd\e82\SHA256E-s20--e394a389d787383843decc5d3d99b6d184ffa5fddeec23b911f9ee7fc8b9ea77\SHA256E-s20--e394a389d787383843decc5d3d99b6d184ffa5fddeec23b911f9ee7fc8b9ea77: DeleteFile "\\\\?\\C:\\Users\\runneradmin\\.t\\tmprepo3\\.git\\annex\\objects\\6cd\\e82\\SHA256E-s20--e394a389d787383843decc5d3d99b6d184ffa5fddeec23b911f9ee7fc8b9ea77\\SHA256E-s20--e394a389d787383843decc5d3d99b6d184ffa5fddeec23b911f9ee7fc8b9ea77": permission denied (Access is denied.)

This is a bit surprising, it seems that chmod -R -w on windows actually
had some effect. But also, on Windows lockContentShared
has to write to a lock file, so it cannot work in a readonly repo.
So git-annex cannot support readonly repos on Windows.

Sponsored-by: Dartmouth College's Datalad project