Bug 1654120: Increase phabricator.test fastcgi buffer size

In a niche situation where a large amount of output is created by Phabricator and a
specific format of output is requested by the client (e.g.: gzip/default encoding), nginx
will choke due to too much data.

According to Stack Overflow [1], this is resolved by increasing the buffer size. Some
trial and error demonstrated that just "fastcgi_buffer_size" needed to be bumped.