Intermittent time-based test failure

I've noticed an intermittent time-based test failure, which probably indicates something is getting the "now" time by default assignment instead of being explicitly assigned in the tests. Something to look into:

```
Scheduler › getNextRequestDelay › returns the same number when a new request that is scheduled later is added

expect(received).toBe(expected) // Object.is equality

Expected: 119964
Received: 119963

261 | const delayAfter = scheduler.getNextRequestDelay( now );
262 |
> 263 | expect( delayAfter ).toBe( delayBefore );
| ^
264 | } );
265 | } );
266 |

at Object.toBe (src/client/__tests__/scheduler.spec.js:263:25)

```

Start by running the failing Scheduler test in src/client/__tests__/scheduler.spec.js and inspect the test around line 263, including how now is assigned. Trace the calls to getNextRequestDelay and make the test deterministic so the expected and received delays remain equal.

Written by the indexing model from the issue text.