test: add explicit test cases for cpu_set_add() and cpu_set_add_range()

cpu_set_add_range() is used in parse_cpu_set(), hence already tested.
But it is better to test these functions explicitly.

For CID#1611787 and CID#1611788, that should be false-positive.

src/shared/cpu-set-util.c

@@ -179,7 +179,7 @@ int cpu_set_add_set(CPUSet *c, const CPUSet *src) {
         return 1;
 }
 
-static int cpu_set_add_range(CPUSet *c, size_t start, size_t end) {
+int cpu_set_add_range(CPUSet *c, size_t start, size_t end) {
         int r;
 
         assert(c);

src/shared/cpu-set-util.h

@@ -24,6 +24,7 @@ void cpu_set_done(CPUSet *c);
 int cpu_set_realloc(CPUSet *c, size_t n);
 int cpu_set_add(CPUSet *c, size_t i);
 int cpu_set_add_set(CPUSet *c, const CPUSet *src);
+int cpu_set_add_range(CPUSet *c, size_t start, size_t end);
 int cpu_set_add_all(CPUSet *c);
 
 char* cpu_set_to_string(const CPUSet *c);