Tweak test case text so it's clearer

  char *line2 = strchr(long_string,'\n')+1;

  char *line3 = strchr(line2,'\n')+1;

  const char *short_string = "hello kitty\n"

    "third line\n";

    "second line\n";

  char *short_line2 = strchr(short_string,'\n')+1;

  (void)ptr;

  test_eq_ptr(line3, find_str_at_start_of_line(long_string, "third line"));

  test_eq_ptr(NULL,  find_str_at_start_of_line(long_string, "third line\n"));

  test_eq_ptr(short_line2, find_str_at_start_of_line(short_string,

                                                     "third line\n"));

                                                     "second line\n"));

 done:

  ;

}