Some fixes in function names and more tests was added.

tests/psubus_i16.ispc

@@ -1,11 +1,11 @@
 
 export uniform int width() { return programCount; }
 
-export void f_f(uniform float RET[], uniform float aFOO[]) {
-    uniform unsigned int8 a = 0, b = 32767; // min and max unsigned int16
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    uniform unsigned int8 a = 0; // min unsigned int16
     RET[programIndex] = saturating_sub(a, b);
 }
 
 export void result(uniform float RET[]) {
-    RET[programIndex] = 0;
+    RET[programIndex] = (uniform unsigned int8) 0;
 }