don't mock windows on windows

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1642487 13f79535-47bb-0310-9956-ffa450edef68
Merged revision(s) 1642488 from lucene/dev/trunk:
LUCENE-5968: Improve error message when 'ant beast' is run on top-level modules
This closes #96

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1642489 13f79535-47bb-0310-9956-ffa450edef68