TEST_BUG: com/sun/jdi/BreakpointWithFullGC.sh fails to cleanup in Cygwin

[Chris Hegarty]

Hi Dan, Alan,

This is a trivial change to use /dev/null instead of NUL for Windows 
test with Cygwin.

http://cr.openjdk.java.net/~chegar/7076756/webrev.00/webrev/

Thanks,
-Chris.