Mock `promptWindowsSpeedUpSites` handler in onboarding unit tests

[fluiddot]

Related to https://github.com/Automattic/studio/pull/211 as it when the handler was introduced.

Proposed Changes

• Mock promptWindowsSpeedUpSites IPC handled to avoid test failures.

Testing Instructions

• Observe that all CI jobs succeed.

Pre-merge Checklist

• [x] Have you checked for TypeScript, React or other console errors?

[fluiddot]

I note we have one other error logged in the test results, but it appears unrelated to the target of the proposed changes.

Yep, I checked the output of recent trunk commits, and seems the error log was introduced in https://github.com/Automattic/studio/pull/203. cc @katinthehatsite