Testsuite: fix test under Octave

Enforce the positivity of standard errors in the maximum likelihood
estimations. Otherwise some stderr are estimated negative, and the Bayesian
estimation fail because the initial value is out of the bounds of the prior
shape.

tests/analytic_derivatives/fs2000_analytic_derivation.mod

@@ -68,8 +68,8 @@ estimated_params;
 alp, 0.356;
 rho, 0.129;
 psi, 0.65;
-stderr e_a, 0.035449;
-stderr e_m, 0.008862;
+stderr e_a, 0.035449, 0, inf;
+stderr e_m, 0.008862, 0, inf;
 end;
 
 varobs gp_obs gy_obs;