'Threads inherit timeLimit from parent Thread' test is flaky

The test `Threads inherit timeLimit from parent Thread` in `server/interpreter_test.js` is flaky, often failing with:

```
got "Thread didn't time out" want 'RangeError: Thread ran too long'
```

Not sure what is going on here. This test often fails, but the similarly-parametered `Thread hits timeLimit` never seems to. Tuning test parameters ought to be able to fix the problem—albeit at the risk of making `Thread can use suspend to avoid timeLimit` flaky, or making the tests run longer than necessary—but the discrepancy between these two similar tests should be investigated in case there is a bug lurking which actually causes timeouts not to be applied to child threads.