Some fixes in function names and more tests was added.

tests/psubs_vi8.ispc

@@ -2,10 +2,10 @@
 export uniform int width() { return programCount; }
 
 export void f_f(uniform float RET[], uniform float aFOO[]) {
-    varying int8 a = -128, b = 127; // min and max unsigned int8
+    varying int8 a = -128, b = aFOO[programIndex]; // min unsigned int8
     RET[programIndex] = saturating_sub(a, b);
 }
 
 export void result(uniform float RET[]) {
-    RET[programIndex] = -128;
+    RET[programIndex] = (varying int8) -128;
 }