fix(pageserver): unique test harness name for merge_in_between

[skyzh]

Problem

As title, there should be a way to detect duplicated harness names in the future :(

Summary of changes

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

[jcsp]

I would be fine with making TenantHarness generate a pseudorandom path in future, this business of hardcoding test names is too error prone

[github-actions]

3054 tests run: 2939 passed, 0 failed, 115 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_ondemand_wal_download_in_replication_slot_funcs: release

Code coverage* (full report)

• functions: 32.7% (6984 of 21338 functions)
• lines: 50.1% (55003 of 109719 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
d0543ee at 2024-07-12T13:30:39.225Z :recycle:}