Add tests for variable proxy_protocol

[ZihaoFU245]

map $ssl_preread_server_name $pp {
example.com on;
example2.com v2;
default off;
}

server {
listen 8000;
proxy_pass backend;
proxy_protocol $pp;
}

Proposed changes

nginx/nginx#1713

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• I have signed the F5 Contributor License Agreement (CLA).
• If applicable, I have added tests that prove my fix is effective or that my feature works.
• If applicable, I have checked that any relevant tests pass after adding my changes.
• I have updated any relevant documentation (README.md and/or CHANGELOG.md).