[Tree] Fix failing unit tests

### Provide a general summary of the issue here

Tree is doing some out-of-sync things wrt scrolling that breaks vitest. We should not be doing those out-of-sync things. 

- Fix `firstUpdated()` being `async`
- Investigate whether `layoutComplete` is also causing problems

### 👤 Your name

Jeroen

### 🧢 Your Product/Team

SLDS