[Bug] jest test helpers is missing ResizeObserver. unobserve

[siddharthkp]

Description

utils/test-helpers.tsx is missing unobserve

Found this while testing it in memex: https://github.com/github/memex/actions/runs/3234611074/jobs/5297929438

After we add this to primer/react, we can remove the local override in memex: https://github.com/github/memex/blob/55746cf4ebfd0b07da6bbee38dcb644257542840/src/tests/jest-setup.ts#L62

Steps to reproduce

Found this while testing it in memex: https://github.com/github/memex/actions/runs/3234611074/jobs/5297929438

Version

v35.11.0

Browser

No response