test: add explicit imports for jest functions

[erikian]

describe, expect and other global Jest types are marked as unresolved on my IDE because we're missing "types": ["jest"].

[dsanders11]

I believe these functions can be imported from @jest/globals which will better align with how it's done in Vitest, which this repo will be converted to after the Node 22 bump.

[erikian]

@dsanders11 agree with having explicit imports, though Jest throws Do not import `@jest/globals` outside of the Jest test environment when importing expect in test/util.ts because that file doesn't have any tests. As a quick workaround, I added a const declaration to that file so we can keep using the injected globals — I'm pretty sure that's not the most elegant way to solve the issue, so suggestions are welcome!