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

tests/check-perf-code.py

@@ -24,7 +24,7 b' perfpypats = ['
 
 def modulewhitelist(names):
     replacement = [('.py', ''), ('.c', ''), # trim suffix
-                   ('mercurial%s' % (os.sep), ''), # trim "mercurial/" path
+                   ('mercurial%s' % ('/'), ''), # trim "mercurial/" path
                   ]
     ignored = {'__init__'}
     modules = {}

tests/test-hgignore.t

@@ -356,7 +356,7 b' Windows paths are accepted on input'
   $ rm dir1/.hgignore
   $ echo "dir1/file*" >> .hgignore
   $ hg debugignore "dir1\file2"
-  dir1\file2 is ignored
+  dir1/file2 is ignored
   (ignore rule in $TESTTMP\ignorerepo\.hgignore, line 4: 'dir1/file*')
   $ hg up -qC .