x-pack/filebeat/input/httpjson: add benchmarks

[efd6]

What does this PR do?

This adds benchmarking for the HTTPJSON input as a prelude to a refactor and revision. It also refactors the structure of the testing code to separate the test table from the test function; this is partly to allow sharing of test cases between the tests and benchmarks, and partly for clarity.

Test names are altered to reduce stutter and make direct copy/paste of the name in terminal output from go test findable in the Go source.

Why is it important?

This is needed to allow a comparison between HTTPJSON and the new CEL input (the original reason), to guard against perf regressions in an upcoming revision, and to allow us to direct performance improvement work based on evidence. The formatting changes in the test table are QoL.

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.

Author's Checklist

• Recommend hide whitespace for review of test file changes.

How to test this PR locally

Run go test in x-pack/filebeat/input/httpjson, optionally with -bench . and friends.

Related issues

• For x-pack/filebeat-cloud / TestInput – github.com/elastic/beats/v7/x-pack/filebeat/input/httpjson tests are flaky #34929

Use cases

Screenshots

Logs

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Build stats

• Start Time: 2023-06-07T16:04:25.489+0000

• Duration: 76 min 45 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2961
Skipped	177
Total	3138

💚 Flaky test report

Tests succeeded.

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