test: Port EventLoopTest to Swift Testing. (apple#3503)

Port `EventLoopTest` to Swift Testing.

### Motivation:

Tests should use the latest Swift Testing framework when possible.

### Modifications:

- Ported `EventLoopTest ` from XCTest to Swift Testing

### Result:

All tests continue to pass, EventLoopTest now has more modern Swift
Testing syntax.

Author: scottmarchant