polish: BusyLineAccessibilityTests read the wall clock and fail on a loaded runner #75

testTheInfobarReadoutLabelFollowsTheTicks (got "1 s", expected "0 s") and testTheBusyLineLabelFollowsTheTicks (got "Disk detection · 2 s", expected "· 0 s") failed on the Xcode 27.1 Mac runner on #70 while the machine was loaded.

Cause: both tests start a wall clock (let start = Date()), then expect the first tick, read after spin(0.3), to say 0 s. When the main thread is late by a second or more between start and the first TimelineView tick, the first tick already says 1 s or 2 s. Reproduced locally by inserting a 1.5 s main-thread stall after start: both fail with exactly the CI messages.

Fix: the tests answer the readouts from a test clock they set (0, then 2), and wait for the tick instead of a fixed sleep; the second check becomes an exact 2 s rather than "not 0 s".