docs: highlight neon env comes with an initial timeline

[skyzh]

Problem

Quite a few existing test cases create their own timelines instead of using the default one. This pull request highlights that and hopefully people can write simpler tests 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

[yliang412]

Thanks. Added some comments related to what I learned from writing my first test.

[github-actions]

3192 tests run: 3053 passed, 0 failed, 139 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_storage_controller_smoke: release

Code coverage* (full report)

• functions: 31.5% (6599 of 20937 functions)
• lines: 48.5% (51047 of 105314 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
c327dc9 at 2024-06-07T20:05:28.423Z :recycle:}