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.

tests/test-blackbox.t

@@ -64,6 +64,8 b' we must not cause a failure if we cannot'
 
   $ hg rollback
   repository tip rolled back to revision 1 (undo pull)
+
+#if unix-permissions
   $ chmod 000 .hg/blackbox.log
   $ hg --debug incoming
   warning: cannot write to blackbox.log: Permission denied
@@ -85,6 +87,7 b' we must not cause a failure if we cannot'
   c
   
   
+#endif
   $ hg pull
   pulling from $TESTTMP/blackboxtest (glob)
   searching for changes
@@ -95,12 +98,13 b' we must not cause a failure if we cannot'
   (run 'hg update' to get a working copy)
 
 a failure reading from the log is fine
-
+#if unix-permissions
   $ hg blackbox -l 3
   abort: Permission denied: $TESTTMP/blackboxtest2/.hg/blackbox.log
   [255]
 
   $ chmod 600 .hg/blackbox.log
+#endif
 
 backup bundles get logged