fix failure of test_pane_order on fedora machines

[marbu]

This pull request fixes the following issue:

When I run unit tests of tmuxp from current master on a fedora machine, test test_pane_order fails like this:

======================================================================
FAIL: test_pane_order (tmuxp.testsuite.workspacebuilder.PaneOrderingTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/martin/projects/tmuxp/tmuxp/testsuite/workspacebuilder.py", line  711, in test_pane_order
    self.assertEqual(p.get('pane_current_path'), pane_path)
AssertionError: u'/usr/sbin' != u'/sbin'
- /usr/sbin
? ----
+ /sbin

This mismatch is likely caused by the fact that /sbin directory is just a link to /bin/sbin (see usr move feature. Since the primary purpose of this unit test is to check the order of panes , I propose to simply use different directory which is not a symlink.

[tony]

@marbu Excellent, thank you!