[ci] fix macos extremely slow demo_directplc_ex1 test

Author: fangq

sample/demo_directplc_ex1.m

@@ -51,7 +51,7 @@
 
 %% mesh generation of the cladding for the side-cut fiber
 % [node,elem,face]=s2m(no,face,1,50);
-[node, elem, face] = surf2mesh(no, fc, min(no), max(no), 1, 50, [0 0 1], [], 0);
+[node, elem, face] = surf2mesh(no, fc, min(no), max(no), 1, 50, [0 0 1], [], 0, 'tetgen1.5');
 
 plotmesh(no, fc, 'y>-0.1');
 % figure;