Some fixes in function names and more tests was added.

tests/psubs_i16.ispc

@@ -1,11 +1,11 @@
 
 export uniform int width() { return programCount; }
 
-export void f_f(uniform float RET[], uniform float aFOO[]) {
-    uniform int16 a = -32768, b = 32767; // min and max signed int16
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    uniform int16 a = -32768; // min signed int16
     RET[programIndex] = saturating_sub(a, b);
 }
 
 export void result(uniform float RET[]) {
-    RET[programIndex] = -32768;
+    RET[programIndex] = (uniform int16) -32768;
 }