Add more substantial tests for compute migrations

[tristan957]

The previous tests really didn't do much. This set should be quite a bit more encompassing.

[tristan957]

One thing worth calling out: is the format of the migration plpsql tests what we want? Would it make more sense to do something like "if neon_superuser is granted access to func_xyz, write a SQL query for func_xyz?"

[github-actions[bot]]

5616 tests run: 5345 passed, 0 failed, 271 skipped (full report)

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Flaky tests (3)

#### Postgres 17 - `test_compute_pageserver_connection_stress`: [debug-x86-64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9811/11922665400/index.html#suites/17ce3111e92c0f109f76121e2725061b/3def6a95d5e776dc/retries) - `test_location_conf_churn[2]`: [debug-x86-64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9811/11922665400/index.html#suites/231aea51ea102d83d3dae52232693454/a38e13b9e6e86625/retries) - `test_sharding_ingest_gaps`: [debug-x86-64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9811/11922665400/index.html#suites/140824de6e814b5b1ae2b622c3f67840/6a7a0ee1bdcc9be3/retries)

Code coverage* (full report)

• functions: 31.4% (7939 of 25248 functions)
• lines: 49.6% (63012 of 127036 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
6fa910d1c9aea142e54ede6987809ef55544c500 at 2024-11-19T23:10:43.941Z :recycle:}