tests: correct the remaining fallout from recent path style changes on Windows Per @martinvonz, `ui.slash` set by the test runner is now capable of playing a more active role.[1] I verified that both of these work by setting `ui.slash` to False, but these changes seem cleaner. The problem with check-perf-code.py was that the proper imports were not being whitelisted due to '\' vs '/'. [1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2019-February/128701.html