chore(neon_test_utils): restrict installation to superuser

[skyzh]

Problem

The test utils should only be used during tests. Users should not be able to create this extension on their own. Create this pull request to see if anything breaks if we only allow installation by superuser.

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

[github-actions]

2886 tests run: 2759 passed, 0 failed, 127 skipped (full report)

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Flaky tests (1)

Postgres 15

• test_gc_aggressive: debug

Code coverage* (full report)

• functions: 31.4% (6242 of 19882 functions)
• lines: 47.1% (46745 of 99299 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
8cf875e at 2024-05-06T18:54:47.175Z :recycle:}