Return to original directory after libbe.bugdir.SimpleBugDirTestCase().

Return to original directory after libbe.bugdir.SimpleBugDirTestCase().

This was causing strange "RCS not found" errors in the bzr and hg
unittests.  For example, the bzr tests all passed:
  wking@thor:be.wtk-rr$ python test.py bzr
  ...
  Ran 12 tests in 24.143s

  OK
Except when run after the bugdir tests:
  wking@thor:be.wtk-rr$ python test.py bugdir bzr
  ...
  Ran 19 tests in 1.862s

  FAILED (errors=12)
Where the failures were all
  AssertionError: bzr RCS not found

Fixed by returning to intial directory after SimpleBugDirTestCase
execution.  Problem is due to Python issues with unlinked directories
though, so bzr/hg will _still_ not work from unlinked directories.
This is for Python 2.5.4 on Ubuntu 8.04.3, but probably effects other
pythons too.

Details:

Isolated problem to unlinked directories:
  mkdir /tmp/a
  cd /tmp/a
  rmdir /tmp/a
  python /home/wking/src/fun/be/be.wtk-rr/test.py bzr
which fails with the same "RCS not found" errors because bzr fails:
  wking@thor:/$ mkdir /tmp/a; cd /tmp/a; rmdir /tmp/a; bzr --help; cd /;
  rmdir: removing directory, /tmp/a
  'import site' failed; use -v for traceback
  bzr: ERROR: Couldn't import bzrlib and dependencies.
  Please check bzrlib is on your PYTHONPATH.

  Traceback (most recent call last):
    File "/usr/bin/bzr", line 64, in <module>
      import bzrlib
  ImportError: No module named bzrlib
which fails becase 'import site' fails:
  wking@thor:/$ mkdir /tmp/a; cd /tmp/a; rmdir /tmp/a; python -c 'import site'; cd /;
  rmdir: removing directory, /tmp/a
  'import site' failed; use -v for traceback
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/home/wking/lib/python/site.py", line 73, in <module>
      __boot()
    File "/home/wking/lib/python/site.py", line 33, in __boot
      imp.load_module('site',stream,path,descr)
    File "/usr/lib/python2.5/site.py", line 408, in <module>
      main()
    File "/usr/lib/python2.5/site.py", line 392, in main
      paths_in_sys = removeduppaths()
    File "/usr/lib/python2.5/site.py", line 96, in removeduppaths
      dir, dircase = makepath(dir)
    File "/usr/lib/python2.5/site.py", line 72, in makepath
      dir = os.path.abspath(os.path.join(*paths))
    File "/usr/lib/python2.5/posixpath.py", line 403, in abspath
      path = join(os.getcwd(), path)
  OSError: [Errno 2] No such file or directory
which fails because our cwd doesn't exist.  That makes sense ;).
Still I think Python should be able to handle it, so I reported it
  http://bugs.python.org/issue6612