Use fcntl.flock() instead of just file existence check, so it won't fail
on a stale lock file. While at it, move locking to a function decorator,
to de-clutter the install function a bit. This will allow reducing
indentation level, but don't do it yet, to make the patch readable.
Move lock testing into a separate test, and remove it from install
tests.
QubesOS/qubes-issues#2534
f1424812b0