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