x-pack/filebeat/input/{cel,httpjson}: fix flaky test

[efd6]

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

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• [ ] 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
• [x] I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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