LaunchpadScript constructor breaks logging for subsequent tests

[Jonathan Lange]

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.

[Robert Collins (lifeless)]

Tests should probably use the FakeLogger fixture from fixtures if they want logging message anyhow; tests that construct a LaunchpadScript should isolate it and restore afterward.