As the values generated by av_bmg_get can be arbitrarily large
(only the stddev is specified), we can't use a fixed tolerance.
Calculate a dynamic tolerance (like in float_dsp from 38f966b2),
based on the individual steps of the calculation.

This fixes running this test with certain seeds, when built with
clang for mingw/x86_32.
Signed-off-by: Martin Storsjö <martin@martin.st>