tests: Fix race condition in tmp webserver
tests: Fix race condition in tmp webserver

I was seeing this when trying to run flatpak's tests in ostree's CI:
https://github.com/ostreedev/ostree/pull/824

The race here is that the python process can still be writing to the output
while sed is reading it, and hence we'll find a difference on the next line.
Fix this by making a tmp copy of the file, which then both sed and cmp will
read consistently.

I'm not *entirely* sure this will fix the problem as I couldn't easily reproduce
the race locally, but I believe it at least fixes *a* race.

GeorgesStavracas/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,flatpak/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak