tests: Accept slightly different numeric results

tests: Accept slightly different numeric results

FLT_EPSILON is the distance between 1.0 and the next distinct floating
point number, and doesn't necessarily have anything to do with the
precision we can expect from a series of floating-point calculations.
Experimentally, 1e-6 is achievable, even on platforms with unusual
floating point implementations like i387.

Resolves: https://gitlab.gnome.org/GNOME/gtk/-/issues/6051
Bug-Debian: https://bugs.debian.org/1050076
Signed-off-by: Simon McVittie <smcv@debian.org>