Add more tests from stringlib

tests/string_strlen.c

@@ -0,0 +1,9 @@
+#include "string.h"
+
+int main(void)
+{
+  if ( strlen("Test") != 4 ) return 1;
+  if ( strlen("Th\0ing") != 2 ) return 2;
+  if ( strlen(NULL) != 0 ) return 3;
+  return 0;
+}