Correct CI check for examples and add a unit test

[wheatear-dev]

This fixes a bug within the CI check for unversioned changes to examples, involving an exclude pattern not matching relevant file paths that contain separators.

What type of PR is this?

Uncomment only one /kind <> line, press enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking

/kind bug

/kind cleanup
/kind documentation
/kind feature
/kind hotfix
/kind release

What this PR does / Why we need it:

My previous PR #3940 - which was merged and is now live - is causing CI pipelines to fail when any changes are made to YAML files in an example, even though these were supposed to have been excluded by a deny list.

This PR provides:

1. A unit test that reproduces the bug (on previous code)
2. A small code fix, that operates the filepath.Match() call on the filename (from filepath.Split()) and not the full path.

Which issue(s) this PR fixes:

Closes #4042

Special notes for your reviewer:

I am very sorry about this, and hope this hasn't left a bad impression.

[gongmax]

/gcbrun

[gongmax]

Thank you for the quick fix! I didn't notice the case when reviewing the code so it's on me too :)

[gongmax]

/gcbrun

[agones-bot]

Build Succeeded 🥳

Build Id: d8e6c6d8-6693-4772-9f4e-20ffd3b18d7c

The following development artifacts have been built, and will exist for the next 30 days:

• image: us-docker.pkg.dev/agones-images/ci/agones-controller:1.46.0-dev-74bde9b
• image: us-docker.pkg.dev/agones-images/ci/agones-extensions:1.46.0-dev-74bde9b
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.46.0-dev-74bde9b-linux
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.46.0-dev-74bde9b
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.46.0-dev-74bde9b
• Linux C++ SDK (build): agonessdk-1.46.0-dev-74bde9b-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.46.0-dev-74bde9b.zip

A preview of the website (the last 30 builds are retained):

• https://74bde9b-dot-preview-dot-agones-images.appspot.com/

To install this version:

git fetch https://github.com/googleforgames/agones.git pull/4045/head:pr_4045 && git checkout pr_4045
helm install agones ./install/helm/agones --namespace agones-system --set agones.image.registry=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.46.0-dev-74bde9b