Fix buildbot regression by rL339929: NameError: global name 'test_directory' is not defined

With buildbot slave under test - I get after rL339929:
http://lab.llvm.org:8014/builders/lldb-x86_64-fedora-28-cmake/builds/243/steps/test1/logs/stdio

  File "/home/buildbot/lldbroot/lldb-x86_64-fedora-28-cmake/scripts/../llvm/tools/lldb/test/dotest.py", line 7, in <module>
    lldbsuite.test.run_suite()
  File "/quad/home/buildbot/lldbroot/lldb-x86_64-fedora-28-cmake/llvm/tools/lldb/packages/Python/lldbsuite/test/dotest.py", line 1177, in run_suite
    configuration.results_formatter_object)
  File "/quad/home/buildbot/lldbroot/lldb-x86_64-fedora-28-cmake/llvm/tools/lldb/packages/Python/lldbsuite/test/dosep.py", line 1692, in main
    dst = core.replace(test_directory, "")[1:]
NameError: global name 'test_directory' is not defined

Patch by Vedant Kumar.

Differential Revision: https://reviews.llvm.org/D51874

llvm-svn: 343726