Update test-nginx library to official git commit 8d5c866836

Updated to official source tree:
   https://github.com/openresty/test-nginx/tree/8d5c8668364251cdae01ccf1ef933d80b642982d

All test cases as following are passed with this updated test-nginx library.
```
tests/test-nginx/cases/footer.t 
tests/test-nginx/cases/proxy_no_buffering_chunked.t 
tests/test-nginx/cases/proxy_no_buffering.t 
tests/test-nginx/cases/syslog.t 
tests/test-nginx/cases/trim.t 
tests/test-nginx/cases/user_agent.t 
tests/test-nginx/cases/limit_upstream_tries.t
```
See this for test log.