tests: avoid some int/size_t conversion size/sign warnings

[monnerat]

Silent the following warnings:

stub_gssapi.c: In function 'gss_init_sec_context':
stub_gssapi.c:212:18: warning: conversion to 'size_t' {aka 'long unsigned int'} from 'int' may change the sign of the result [-Wsign-conversion]
  212 |   token = malloc(length);
      |                  ^~~~~~
stub_gssapi.c:222:26: warning: conversion to 'size_t' {aka 'long unsigned int'} from 'int' may change the sign of the result [-Wsign-conversion]
  222 |   used = snprintf(token, length, "%s:%s:%d:", creds,
      |                          ^~~~~~
stub_gssapi.c:233:36: warning: conversion to 'size_t' {aka 'long unsigned int'} from 'int' may change the sign of the result [-Wsign-conversion]
  233 |   memset(token + used, 'A', length - used);
      |                             ~~~~~~~^~~~~~
stub_gssapi.c:238:26: warning: conversion to 'size_t' {aka 'long unsigned int'} from 'int' may change the sign of the result [-Wsign-conversion]
  238 |   output_token->length = length;
      |                          ^~~~~~

As a side effect, it also fixes an snprintf() error detection.

[bagder]

Thanks!