tests: make Workload more determinstic

[jcsp]

Problem

Previously, Workload was reconfiguring the compute before each run of writes, which was meant to be a no-op when nothing changed, but was actually writing extra data due to an issue being fixed in #10696.

The row counts in tests were too low in some cases, these tests were only working because of those extra writes that shouldn't have been happening, and moreover were relying on checkpoints happening.

Summary of changes

• Only reconfigure compute if the attached pageserver actually changed. If pageserver is set to None, that means controller is managing everything, so never reconfigure compute.
• Update tests that wrote too few rows.

[github-actions]

7436 tests run: 7056 passed, 0 failed, 380 skipped (full report)

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Flaky tests (2)

Postgres 17

• test_compute_installed_extensions_metric: release-arm64-without-lfc

Postgres 16

• test_s3_eviction[0.2-False]: release-arm64-with-lfc

Code coverage* (full report)

• functions: 33.2% (8583 of 25872 functions)
• lines: 49.1% (72278 of 147344 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
c765718 at 2025-02-12T11:23:00.790Z :recycle:}

[jcsp]

Failures due to stuck tests: {'test_sharding_split_failures[release-pg15-failure10]', 'test_sharding_split_failures[release-pg15-failure5]'}

Different runs have various different permutations of sharding_split_failures getting stuck.