test: fix on demand activation test flakyness

[VladLazar]

Warm-up (and the "tenant startup complete" metric update) happens in
a background tokio task. The tenant map is eagerly updated (can happen
before the task finishes).

The test assumed that if the tenant map was updated, then the metric
should reflect that. That's not the case, so we tweak the test to wait
for the metric.

Fixes #7158

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]

2706 tests run: 2574 passed, 0 failed, 132 skipped (full report)

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Code coverage* (full report)

• functions: 28.3% (7130 of 25197 functions)
• lines: 46.9% (43711 of 93289 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
c16dbf9 at 2024-03-20T10:04:58.256Z :recycle:}

[VladLazar]

180 runs of the test without any failures in https://neon-github-public-dev.s3.amazonaws.com/reports/pr-7180/8348245330/index.html