Fix two tests that only pass under Apple's XCTest

[lhoward]

Description

Two test-only changes, no library code touched:

• Tests/AsyncSubjets/StreamedTests.swift imports Combine but uses nothing from it; the import stops the test target compiling wherever Combine is absent. Removed.
• AsyncLazySequenceTests.test_AsyncLazySequence_returns_an_asyncSequence_that_finishes_when_task_is_cancelled is a synchronous test that returns while the Task it started is still await fulfillment(of:)-ing an expectation. Apple's XCTest tolerates that; corelibs-XCTest reports Failed due to unwaited expectation 'The task has been cancelled'. The test is now async, uses await fulfillment(of:) for the first expectation and awaits the task before returning, so the assertion inside the task also runs before the test ends.

Tests

Linux, Swift 6.4, x86_64, with #70 applied underneath (the library needs it to compile here), three consecutive runs:

Executed 199 tests, with 0 failures (0 unexpected) in 15.61 (15.61) seconds
Executed 199 tests, with 0 failures (0 unexpected) in 15.478 (15.478) seconds
Executed 199 tests, with 0 failures (0 unexpected) in 15.672 (15.672) seconds

Checklist

• this PR is based on the main branch and is up-to-date
• the commits inside this PR have explicit commit messages
• unit tests cover the new feature or the bug fix (test-only change)
• the CHANGELOG is up-to-date (no user-visible change)

🤖 Generated with Claude Code

https://claude.ai/code/session_01T8cahEd4GfsHQ89sNrmP4p

[twittemb]

Closing as superseded by merged PR #74: #74.

Both fixes are now on main: the unused Combine import has been removed, and the lazy-sequence cancellation test is asynchronous, awaits the first expectation, and awaits the child task before returning. The full 200-test suite passes in Linux CI on Swift 6.1.3 and 6.4.0, so this PR no longer needs to be merged.