Skip test if "nan" package isn't loaded

--- biosig4octave-2.4.0/inst/t400_Classification/sumskipnan.m.orig	2022-03-19 12:21:44.000000000 +0100
+++ biosig4octave-2.4.0/inst/t400_Classification/sumskipnan.m	2022-03-19 19:13:19.308144348 +0100
@@ -188,4 +188,7 @@
 %!assert(sumskipnan([1,NaN],2),1)
 %!assert(sumskipnan([1,NaN],2),1)
 %!assert(sumskipnan([nan,1,4,5]),10)
-%!assert(sumskipnan([nan,1,4,5]',1,[3;2;1;0]),6)
+%!test
+%! if (exist('sumskipnan_mex'))
+%!   assert(sumskipnan([nan,1,4,5]',1,[3;2;1;0]),6);
+%! end