Fix failing SnippetsFilter functional test

[bjee19]

SnippetsFilter functional test has been failing frequently, which has been blocking our other pipeline runs.

See the following failing pipeline runs:

https://github.com/nginx/nginx-gateway-fabric/actions/runs/16995932607/job/48186889345?pr=3750
https://github.com/nginx/nginx-gateway-fabric/actions/runs/16995932607/job/48186889311?pr=3750
https://github.com/nginx/nginx-gateway-fabric/actions/runs/17051707634/job/48341189295?pr=3742
https://github.com/nginx/nginx-gateway-fabric/actions/runs/17051707634/job/48341189271?pr=3742
https://github.com/nginx/nginx-gateway-fabric/actions/runs/17051707634/job/48341189286?pr=3742

[bjee19]

See this comment in the closing PR: #3751 (comment) for more details. There is one more issue regarding the nginx worker process taking a long time to shut down which is still under investigation.