test: add UT for filewatcher

[Zhupku]

This pull request introduces several changes to the pkg/filewatcher package to improve the handling of file watching and add tests. The most important changes include the addition of a mockable exit function, the introduction of a reset function for testing purposes, and the creation of new tests for the file watcher functionality.

Changes to pkg/filewatcher/filewatcher.go:

• Added a mockable exit function to handle program termination, replacing direct calls to os.Exit with exit in the checkForFileChanges function. [1] [2]
• Introduced a resetWatchCertificateFileOnce function to reset the watchCertificateFileOnce variable for testing purposes.
• Removed an unnecessary goroutine in the WatchFileForChanges function to simplify the code.

Changes to pkg/filewatcher/filewatcher_test.go:

• Added a new test file with tests for the WatchFileForChanges function, including tests for existing and non-existent files.<!-- Thanks for sending a pull request! Here are some tips for you:

1. If this is your first time, please read our contributor guidelines: https://git.k8s.io/community/contributors/guide#your-first-contribution and developer guide https://git.k8s.io/community/contributors/devel/development.md#development-guide
2. Please label this pull request according to what type of issue you are addressing, especially if this is a release targeted pull request. For reference on required PR/issue labels, read here:
   https://git.k8s.io/community/contributors/devel/release.md#issue-kind-label
3. Ensure you have added or ran the appropriate tests for your PR: https://git.k8s.io/community/contributors/devel/testing.md
4. If you want faster PR reviews, read how: https://git.k8s.io/community/contributors/guide/pull-requests.md#best-practices-for-faster-reviews
5. Follow the instructions for writing a release note: https://git.k8s.io/community/contributors/guide/release-notes.md
6. If the PR is unfinished, see how to mark it: https://git.k8s.io/community/contributors/guide/pull-requests.md#marking-unfinished-pull-requests
   -->

What type of PR is this?

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Special notes for your reviewer:

Release note:

none

[k8s-ci-robot]

Hi @Zhupku. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[andyzhangx]

/lgtm
/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andyzhangx, Zhupku

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [andyzhangx]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[andyzhangx]

/retest

[Zhupku]

/retest