Add comprehensive memory leak tests for PassthroughRelay

- Test subscription release with different initialization orders
- Test withLatestFrom operator scenarios
- Mirrors test coverage from CurrentValueRelay (PR #137)
- Verifies fix for issue #167

Remove exported Foundation import to fix DocC builds

CombineExt 1.8.0

CombineExt 1.6.0

CombineExt 1.5.1

Subs in a composed form of withLatestFrom and adds test cases for lon…

…e upstream completions.

CombineExt 1.3.0

CombineExt 1.2.0