Added primitive unit testing in tests/

tests/stdlib_atoi.c

@@ -0,0 +1,11 @@
+#include "stdlib.h"
+
+#define NULL 0x00
+
+int main(void)
+{
+  if ( atoi("1234\0") != 1234 ) return 1;
+  if ( atoi("65536\0") != 65536 ) return 2;
+  if ( atoi("-32767\0") != -32767 ) return 3;
+  return 0;
+}