blackbox: don't run permission tests on non-unix systems

The windows and vfat test runs were failing due to read/write permissions not
working the same on those systems.  On vfat, permissions can't be changed
at all, and on windows it seems the chmod emulation doesn't remove read
permissions.  We could theoretically get the 'cannot write to blacklog.log'
test to pass on windows but there's no #if condition to let us exclude vfat
only.

Verified that test-blackbox passes on windows now.

--HG--
branch : stable