tests: use semaphore instead of lock for Endpoint.running

[jcsp]

Problem

Ahem, let's try this again.

https://github.com/neondatabase/neon/pull/8110 had a spooky failure in test_multi_attach where a call to Endpoint.stop() timed out waiting for a lock, even though we can see an earlier call completing and releasing the lock. I suspect something weird is going on with the way pytest runs tests across processes, or use of asyncio perhaps.

Anyway: the simplest fix is to just use a semaphore instead: if we don't lock we can't deadlock.

Summary of changes

• Make Endpoint.running a semaphore, where we add a unit to its counter when starting the process and atomically decrement it when stopping.

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[bot]]

3228 tests run: 3111 passed, 0 failed, 117 skipped (full report)

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Flaky tests (1)

#### Postgres 15 - `test_timeline_size_quota_on_startup`: [release](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-8112/9583413584/index.html#suites/c39429f093f87547b2a3b0943e2522d9/e8b31b0f2f249715/retries)

Code coverage* (full report)

• functions: 32.4% (6848 of 21165 functions)
• lines: 49.7% (53399 of 107360 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
8cc4c9d1bf9d0a56772f138cb31771b660fc619f at 2024-06-19T15:46:27.823Z :recycle:}