Add safekeeper scrubber test

[arpad-m]

The test is very rudimentary, it only checks that before and after tenant deletion, we can run scan_metadata for the safekeeper node kind. Also, we don't actually expect any uploaded data, for that we don't have enough WAL (needs to create at least one S3-uploaded file, the scrubber doesn't recognize partial files yet).

The scan_metadata scrubber subcommand is extended to support either specifying a database connection string, which was previously the only way, and required a database to be present, or specifying the timeline information manually via json. This is ideal for testing scenarios because in those, the number of timelines is usually limited.

[github-actions]

3827 tests run: 3721 passed, 0 failed, 106 skipped (full report)

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Flaky tests (4)

Postgres 16

• test_change_pageserver: debug-x86-64
• test_lfc_resize: debug-x86-64
• test_subscriber_restart: debug-x86-64

Postgres 15

• test_local_file_cache_unlink: release-x86-64

Code coverage* (full report)

• functions: 32.5% (7416 of 22827 functions)
• lines: 50.6% (60049 of 118663 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
666295e at 2024-08-30T23:08:09.611Z :recycle:}

[skyzh]

LGTM, @yliang412 might want to add this to the cron job if necessary?

[yliang412]

LGTM, @yliang412 might want to add this to the cron job if necessary?

I think we can do it if the checks are stable + do not emit false positive. As part of #8128, I only checked into pageserver's metadata scan code.