Start separating the script runner from the test case.
* bzrlib/tests/test_script.py:
Miniaml modifications to make the tests pass again.
* bzrlib/tests/script.py:
More doc.
(ScriptRunner): Extracted from TestCaseWithScript to better
highlight the TestCase requirements.
(TestCaseWithTransportAndScript): We requires a disk based test
case so far.