Move conformance tests to tests directory

[lucacome]

Proposed changes

Problem: The conformance directory is outside of our tests directory containing all of our tests. This conformance directory has a separate Makefile with many duplicated commands and variables.
At release time we have to update all the versions in multiple places.

Solution: Move the conformance tests inside the tests directory and consolidate the Makefile so it's easier to maintain and update at release time

Closes #1977

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

NONE

[ciarams87]

Looks good overall!

There's just a couple of developer docs that need to be updated as well:

https://github.com/nginxinc/nginx-gateway-fabric/blob/main/docs/developer/quickstart.md?plain=1#L198

https://github.com/nginxinc/nginx-gateway-fabric/blob/main/docs/developer/release-process.md?plain=1#L39

https://github.com/nginxinc/nginx-gateway-fabric/blob/main/docs/developer/testing.md?plain=1#L106

[lucacome]

Thanks for catching those @ciarams87 ! I have updated them.

[ciarams87]

LGTM!

[sjberman]

https://github.com/kubernetes-sigs/gateway-api/tree/main/conformance/reports/v1.0.0/nginxinc-nginx-gateway-fabric will need its README updated (as well as when we introduce our next release that supports v1.1.0).