Exception during testbuild leaves un-removable directory.

[mikeferguson]

I reset the buildbot during a testbuild, when it restarted the new test failed as the /tmp/X_testbuild directory was still owned by root.

Traceback (most recent call last):
  File "/runscript", line 238, in <module>
    run_build_and_test(sys.argv[1], sys.argv[2])
  File "/runscript", line 49, in run_build_and_test
    os.makedirs(workspace+'/build')
  File "/usr/lib/python2.7/os.py", line 157, in makedirs
    mkdir(name, mode)
OSError: [Errno 17] File exists: '/tmp/x_testbuild/build'