make three timing-dependent tests assert behaviour instead of wall-clock bounds

[mauricecarrier7]

What

Three toolkit tests failed on CI because of wall-clock timing, not because of a code defect. Each one now waits for the event it is about, or uses an injected clock.

Test	What it depended on	Now
AudiobookNetworkServiceTest.testDownloadProgressWithEmptyTracks	A 1 s asyncAfter on main had to fire within a 2 s wait. On iOS 26.5 it did not (runs of 8.0 s and 17.9 s).	Waits on whenPendingWorkDrains, a new internal hook on DefaultAudiobookNetworkService that runs a block after the queue's pending barriers and their main-queue hops. It then asserts that nothing was published.
AudiobookPlaybackModelAutosaveTests.test_saveSuppression_whenItLapses_theNextDueTickWrites	20 s of simulated ticks had to be delivered inside a real 0.4 s suppression window.	AudiobookPlaybackModel reads the window through wallClock, which defaults to Date(). The test pins that clock while the window holds and then advances it.
LCPResourceLoaderStallTests.testContentInfo_WhenLengthLookupFailsLate_FinishesWithoutLength	A 0.4 s lookup had to outlast a 0.1 s stall timeout. Its 2-byte range also raced the data phase's 0.1 s timer. It failed after 37.7 s on ios-core's macos-15 run 36911638959.	The lookup parks until the test releases it. The test first checks that the request is still open after five stall timeouts. The request asks for content information only, and the final wait is a 30 s hang bound.

Why

ios-core is moving CI to Xcode 26.6 and iOS 26.5, and its toolkit-tests job failed on these tests. The third test also failed on the current macos-15 runner, so it is not specific to iOS 26.

Production changes

Two small seams, both with unchanged default behaviour:

• DefaultAudiobookNetworkService.whenPendingWorkDrains(_:): internal and only called by tests.
• AudiobookPlaybackModel.wallClock: defaults to Date(). Only the save-suppression window reads it. The other suppression windows still call Date() directly.

Verification

• Full suite, run locally on an iOS 26.2 simulator with Xcode 26.3: Executed 315 tests, with 1 test skipped and 0 failures, ** TEST SUCCEEDED **.
• The three tests ran 10 iterations each (30 executions, 0 failures).
• Each fixed test was checked against a deliberately broken version of the behaviour it covers. All five changes made it fail on a named assertion:
  • The empty service publishes .downloadComplete at startup: the network test fails because it received [downloadComplete].
  • A suppressed tick moves the autosave baseline: XCTUnwrap fails because the first due tick did not write.
  • The suppression check is removed: the premise assertion fails.
  • A stall timer is added to the content-information phase: XCTAssertNil(outcome) fails with transferStalled.
  • A failed lookup reports a length of 0: XCTAssertNil(contentLength) fails.

Not done: the other suppression windows in AudiobookPlaybackModel still read Date() directly, and I did not change any other wall-clock tests in the suite.