tests: rutime: out_http: simplify and make the in_http test more robust.

[pwhelan]

Summary

This PR simplifies the in_http test for out_http, designed to make sure the defaults work well together, to use a single fluent-bit context. This should hopefully make it work better on machines with higher occupancy.

This PR also improves the chances of success by using a try loop to detect when the output is finally flushed.

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Testing
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• Run local packaging test showing all targets (including any new ones) build.
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Documentation

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Backporting

• Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.