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

--- a/tests/check-perf-code.py	Wed Feb 13 18:34:08 2019 -0800
+++ b/tests/check-perf-code.py	Sun Feb 17 22:39:12 2019 -0500
@@ -24,7 +24,7 @@
 
 def modulewhitelist(names):
     replacement = [('.py', ''), ('.c', ''), # trim suffix
-                   ('mercurial%s' % (os.sep), ''), # trim "mercurial/" path
+                   ('mercurial%s' % ('/'), ''), # trim "mercurial/" path
                   ]
     ignored = {'__init__'}
     modules = {}