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.