TestProgram uses defaultTestLoader to load tests. Calling discover() on a TestLoader sets _top_level_dir. Calling discover with no top_level_dir implictly sets _top_level_dir but only the first time: the second time it is called with a different start_dir the prior one is used.

This leads to errors like:
======================================================================
ERROR: test_run_list_failed_import (testtools.tests.test_run.TestRun)
testtools.tests.test_run.TestRun.test_run_list_failed_import
----------------------------------------------------------------------
_StringException: Traceback (most recent call last):
  File "/home/robertc/personal/testtools/testtools/tests/test_run.py", line 175, in test_run_list_failed_import
    run.main, ['prog', 'discover', '-l', broken.package.base, '*.py'], out)
  File "/home/robertc/personal/testtools/testtools/testcase.py", line 420, in assertRaises
    self.assertThat(our_callable, matcher)
  File "/home/robertc/personal/testtools/testtools/testcase.py", line 431, in assertThat
    mismatch_error = self._matchHelper(matchee, matcher, message, verbose)
  File "/home/robertc/personal/testtools/testtools/testcase.py", line 481, in _matchHelper
    mismatch = matcher.match(matchee)
  File "/home/robertc/personal/testtools/testtools/matchers/_exception.py", line 108, in match
    mismatch = self.exception_matcher.match(exc_info)
  File "/home/robertc/personal/testtools/testtools/matchers/_higherorder.py", line 62, in match
    mismatch = matcher.match(matchee)
  File "/home/robertc/personal/testtools/testtools/testcase.py", line 412, in match
    reraise(*matchee)
  File "/home/robertc/personal/testtools/testtools/matchers/_exception.py", line 101, in match
    result = matchee()
  File "/home/robertc/personal/testtools/testtools/testcase.py", line 965, in __call__
    return self._callable_object(*self._args, **self._kwargs)
  File "/home/robertc/personal/testtools/testtools/run.py", line 413, in main
    stdout=stdout)
  File "/home/robertc/personal/testtools/testtools/run.py", line 208, in __init__
    self.parseArgs(argv)
  File "/home/robertc/personal/testtools/testtools/run.py", line 252, in parseArgs
    self._do_discovery(argv[2:])
  File "/home/robertc/personal/testtools/testtools/run.py", line 368, in _do_discovery
    loaded = loader.discover(self.start, self.pattern, self.top)
  File "/home/robertc/.virtualenvs/testtools-2.7/local/lib/python2.7/site-packages/unittest2/loader.py", line 353, in discover
    raise ImportError('Start directory is not importable: %r' % start_dir)
ImportError: Start directory is not importable: '/tmp/tmpac4uwI'


When a test tries to test the behaviour of TestProgram (or a subclass thereof).

The fix is straight forward: at the end of discover restore the value of _top_level_dir that it had at entry.