add trivial compare-and-swap test

test/cas.c

@@ -0,0 +1,23 @@
+#include "CAS.h"
+#include <assert.h>
+
+int main() {
+  size_t x;
+  x=0;
+  assert((compare_and_swap(&x,1,2)==0 , x==0));
+  assert((compare_and_swap(&x,0,2)==1 , x==2));
+
+  assert((atomic_add(&x,5) , x == 7));
+  assert((atomic_add(&x,-5) , x == 2));
+
+  assert(atomic_add_return(&x,5) == 7);
+  assert(atomic_add_return(&x,-5) == 2);
+
+  atomic_inc(&x); assert(x==3);
+  assert(atomic_inc_return(&x) == 4);
+  atomic_dec(&x); assert(x==3);
+  assert(atomic_dec_return(&x) == 2);
+
+  assert((atomic_or(&x,1) , x==3));
+  assert((atomic_and(&x,~1) , x==2));
+}