[7.17](backport #41358) x-pack/filebeat/input/{cel,httpjson}: fix flaky test

[mergify]

Proposed commit message

Do not use uncontrolled randomisation in tests where it is not necessary. The retry tests in the httpjson and cel packages were using a randomised 5xx HTTP status code to trigger the retry behaviour of the the go-retryablehttp package. This had the unfortunate consequence of causing 2% of test runs to fail.

The reason for this is given in the go-retryablehttp documentation[1]:

Mainly, if an error is returned by the client (connection errors, etc.), or if a 500-range response code is received (except 501), then a retry is invoked after a wait period.

Since the package is already tested, and is documented to accept all 5xx status codes except 501 to cause a retry, just use 500.

[1]https://pkg.go.dev/github.com/hashicorp/[email protected]#section-readme

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

Author's Checklist

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How to test this PR locally

Related issues

• For x-pack/filebeat: AWS test failure #40503

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Logs

---

This is an automatic backport of pull request #41358 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 794b832 has failed:

On branch mergify/bp/7.17/pr-41358
Your branch is up to date with 'origin/7.17'.

You are currently cherry-picking commit 794b832932.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   CHANGELOG-developer.next.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   x-pack/filebeat/input/cel/input_test.go
	both modified:   x-pack/filebeat/input/httpjson/input_test.go

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