assert is for comparison. Your code is checking if the marker of the found line is an asterisk. It is not, so the result is the assertion fails.
Your asterisk was created by scatter, so is a scatter object. To find it, your type should be 'scatter'.
t = linspace(0,2*pi,100);
y = cos(t);
m = plot_cos(y, t)
h = findobj(gcf,'Type', 'Scatter');
assert(strcmp(h.Marker, '*'));
function m = plot_cos(y, t)
figure;
plot(t, y, 'b--'); hold on;
[m,I] = min(y);
s = scatter(t(I),m);
s.MarkerEdgeColor = 'r';
s.Marker = '*';
end
