tests: simplify code around unstable test_basebackup_with_high_slru_count

[jcsp]

Problem

In test_basebackup_with_high_slru_count, the pageserver is sometimes mysteriously hanging on startup, having been started+stopped earlier in the test setup while populating template tenant data.

• Investigate test_basebackup_with_high_slru_count failure #7586

We can't see why this is hanging in this particular test. The test does some weird stuff though, like attaching a load of broken tenants and then doing a SIGQUIT kill of a pageserver.

Summary of changes

• Attach tenants normally instead of doing a failpoint dance to attach them as broken
• Shut the pageserver down gracefully during init instead of using immediate mode
• Remove the "sequential" variant of the unstable test, as this is going away soon anyway
• Log before trying to acquire lock file, so that if it hangs we have a clearer sense of if that's really where it's hanging. It seems like it is, but that code does a non-blocking flock so it's surprising.

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]

3213 tests run: 3080 passed, 0 failed, 133 skipped (full report)

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Flaky tests (1)

Postgres 15

• test_compaction_induced_by_detaches_in_history: release

Code coverage* (full report)

• functions: 32.6% (7003 of 21497 functions)
• lines: 50.0% (55265 of 110587 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
189a097 at 2024-07-24T08:34:14.803Z :recycle:}