Add functional tests for ProxySettingsPolicy #4264
Description
As an NGF developer
I want functional tests for the ProxySettingsPolicy
So that I can ensure that nginx behaves properly when defining proxy settings.
Acceptance:
- Functional tests are added for ProxySettingsPolicy
- https://github.com/nginx/nginx-gateway-fabric/blob/main/docs/proposals/proxy-settings.md#testing
- verify nginx config looks correct
Dev notes
- Existing functional tests for other policies can be used for reference