Fix scripttest module bootstrap problem with self_test.py

[franklin42]

self_test.py has some code to install the scripttest package using
pip. However, self_test.py imports from pkgtest, which in turn
imports from scripttest, failing before it can actually do the
install. One must manually 'pip install scripttest' to get self_test.py to work.

Only a few things from scripttest are used by pkgtest. Those should be factored out into a separate module.