docs: remove unncessary use of `async`

[DaleSeo]

What does this PR do?

The async keyword is unnecessary and could be misleading because there is no await calls in the test functions.

• [x] Documentation or TypeScript types (it's okay to leave the rest blank in this case)
• [ ] Code changes

How did you verify your code works?

[paperclover]

wonder if we should have an example using async and await since it is supported

[DaleSeo]

@paperdave I found some examples using async on https://bun.sh/docs/test/writing#basic-usage. I have no opinions but I guess this is sufficient?