Add `@MainActor` to `waitForExpectations(timeout:handler:)`

[fumoboy007]

The Darwin implementation of XCTest added @MainActor to waitForExpectations(timeout:handler:). The open-source implementation should do the same.

[fumoboy007]

Hmm I also tried to use the existing waitForExpectations(timeout:handler:) API without await. It fails with the following on Linux:

error: Exited with signal code 5

[fumoboy007]

Maybe related: https://c.im/@leogdion/109791519876656548