tests: reinstate some syntethic size tests

[VladLazar]

Problem

test_empty_tenant_size was marked xfail and a few other tests were skipped.

Summary of changes

Stabilise test_empty_tenant_size. This test attempted to disable checkpointing for the postgres instance
and expected that the synthetic size remains stable for an empty tenant. When debugging I noticed that
postgres was issuing a checkpoint after the transaction in the test (perhaps something changed since the
test was introduced). Hence, I relaxed the size check to allow for the checkpoint key written on the pageserver.

Also removed the checks for synthetic size inputs since the expected values differ between postgres versions.

Closes #7138

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]

2748 tests run: 2624 passed, 0 failed, 124 skipped (full report)

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Code coverage* (full report)

• functions: 27.9% (6382 of 22848 functions)
• lines: 46.8% (44992 of 96049 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
c2cb843 at 2024-04-04T09:59:35.586Z :recycle:}

[VladLazar]

test_empty_tenant_size is writing a checkpoint key after the initial size is fetched. Looking into where that's coming from.

[VladLazar]

Stabilised test_empty_tenant_size and did 50 repeats on all other previously skipped tests (report). Looking good.

[koivunej]

Looks good, and I like the explanation.