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
#require serve
$ hg clone http://localhost:$HGPORT/ copy
abort: * (glob)
[255]
$ test -d copy
[1]
$ "$PYTHON" "$TESTDIR/dumbhttp.py" -p $HGPORT --pid dumb.pid
$ cat dumb.pid >> $DAEMON_PIDS
$ hg clone http://localhost:$HGPORT/foo copy2
abort: HTTP Error 404: * (glob)
[255]
$ killdaemons.py