LaunchpadScript constructor breaks logging for subsequent tests

LaunchpadScript changes the global root logger to print any logged messages to stderr. While this is great for actual scripts, it means that whenever we invoke LaunchpadScript in a test run, any subsequent tests that log to *any* logger that is set to propagate (the default) will print to stderr.

This is ugly and makes debugging harder by adding extraneous noise.

An example of this is gettext_po_parser.txt, which emits nothing when run by itself, but emits to stderr during a full test run.