tests: Tolerate greater rendering differences on all non-x86_64
Previously, this used 32-bit vs. 64-bit to discriminate, with 32-bit architectures allowed to deviate more from the reference rendering (mainly because the i387 FPU used on baseline i386 builds has excess precision in its FP registers). However, one test also produces a colour channel difference of 3 on Debian mips64el, a 64-bit architecture. Tolerate this too.
Signed-off-by: Simon McVittie email@example.com