Add tests for stdlib

tests/stdlib_isxdigit.c

@@ -0,0 +1,19 @@
+#include "stdlib.h"
+
+#define NULL 0x00
+
+int main(void)
+{
+  const char *postest = "ABCDEFabcdef0123456789";
+  const char *negtest = "GHIJKLMNOPQRSTUVWXYZghijklmnopqrstuvwxyz~!@#$%^&*()_+{}|[]\\:\";'<>?,./`/*-.";
+  char *ptr = NULL;
+
+  for ( ptr = (char *)postest; *ptr != '\0' ; ptr++ ) {
+    if ( isxdigit((int)*ptr) != 1 ) return (int)(ptr - postest);
+  }
+  for ( ptr = (char *)negtest; *ptr != '\0' ; ptr++ ) {
+    if ( isxdigit((int)*ptr) != 0 ) return (int)(100 + (ptr - negtest));
+  }
+
+  return 0;
+}