tests: make doctests not depend on str(ParseError()) format `ParseError` implements `__bytes__`, but it doesn't implement `__str__`, so it gets the default `__str__` implementation. The next patch will make it so `ParseError` gets a `__str__` implementation, which changes the format slightly. This prepares by making us not depend on the format. Differential Revision: https://phab.mercurial-scm.org/D9349