tests: stabilize test_sharding_split_compaction

[jcsp]

Problem

This test incorrectly assumed that a post-split compaction would only drop content. This was easily destabilized by any changes to image generation rules.

In this, and other tests that explicitly create image layers (e.g. latest test_pg_regress & test_isolation), there is also a bug in compaction with force-image-layers, where it will fail trying to create a duplicate image layer.

Summary of changes

• Before split, do a full image layer generation pass, to guarantee that post-split compaction should only drop data, never create it.
• Fix the force_image_layer_creation mode of compaction that we use from tests like this: previously it would try and generate image layers even if one already existed with the same layer key, which caused compaction to fail.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

3042 tests run: 2927 passed, 0 failed, 115 skipped (full report)

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Flaky tests (4)

Postgres 16

• test_ondemand_wal_download_in_replication_slot_funcs: release
• test_pg_regress[None]: debug

Postgres 15

• test_peer_recovery: debug

Postgres 14

• test_isolation[None]: debug

Code coverage* (full report)

• functions: 32.6% (6938 of 21302 functions)
• lines: 50.0% (54549 of 109206 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
e8c1c87 at 2024-07-10T09:40:12.887Z :recycle:}