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)

tests/kalman/likelihood_from_dynare/fs2000ns_uncorr_ME.mod

 @#include "fs2000ns_model.inc" 
 
+% Under Octave, the default seed 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).
+if isoctave
+    set_dynare_seed(2);
+end
+
 stoch_simul(periods=200, order=1,irf=0);
 temp=oo_.endo_simul;
 %add measurement error