tests: wrong pid path when running jobs greater than 1. (#1961)

Author: zhuizhuhaomeng

t/157-socket-keepalive-hup.t

@@ -32,7 +32,8 @@ __DATA__
  set $port $TEST_NGINX_SERVER_PORT;
 
  content_by_lua_block {
- local f, err = io.open("$TEST_NGINX_SERVER_ROOT/logs/nginx.pid", "r")
+ local pidfile = ngx.config.prefix() .. "/logs/nginx.pid"
+ local f, err = io.open(pidfile, "r")
  if not f then
  ngx.say("failed to open nginx.pid: ", err)
  return