Added a main() to atomic.c

test/atomic.c

@@ -1,3 +1,5 @@
+#include <stdio.h>
+
 #include "ucore/atomic.h"
 UC_ATOMIC(unsigned int) i;
 
@@ -48,3 +50,20 @@ unsigned int get_i_compbar()
 }
 
 
+int main(int argc, char *argv[])
+{
+    printf("i: %u\n", get_i());
+    inc_i();
+    printf("i: %u\n", get_i());
+    inc_i();
+    dec_i();
+    printf("i: %u\n", get_i());
+    add_i(100);
+    printf("i: %u\n", get_i());
+    sub_i(50);
+    printf("i: %u\n", get_i_membar());
+
+
+    return 0;
+}
+