test-concurrency: Explicitly use floor division
test-concurrency: Explicitly use floor division

Python 3 is pickier about this. Python 2.7 has Python 3-compatible
semantics for division when the division feature is imported from the
future.

Signed-off-by: Simon McVittie <smcv@debian.org>

Closes: #1457
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree