Test disabling proxy_request_buffering to make LFS work

LFS on GitLab.com doesn't work when the files are large enough (e.g. > 300-400 MB). One way to fix this is to disable proxy_request_buffering:

• gitlab-org/omnibus-gitlab#945 (closed)
• https://gitlab.com/gitlab-org/gitlab-ce/issues/31871#note_29480359

This entry is required in /etc/gitlab/gitlab.rb:

nginx['custom_gitlab_server_config'] = 'proxy_request_buffering off;'