Testsuite: change the seed for some Octave test

Under Octave, the default seed in fs2000ns_uncorr_ME.mod leads to a generated
dataset that makes fs2000ns_corr_ME.mod and fs2000ns_corr_ME_missing.mod
fail (due to a large difference between univariate and multivariate diffuse
filters).

(cherry picked from commit 633f1ad8)