Flake batch 20: eviction idle wait, startup-step deadline backstop, blob GC test budget
per_checkout_semantic_runtime::views_off_search_after_idle_eviction_reports_the_index_reloading failed on macOS in #1103's merge-queue group with "the root must be idle": it evicted once, right after readiness, while the root's background work was still finishing, and eviction refuses a root that isn't idle yet. It now retries eviction until it's accepted, within the file's 90 s deadline, the same way the views-on sibling test already waits for its lane. Test-only; the test passed 3 times on Linux, and the Windows target check passes.
Summary by cubic
Fixes three flaky tests on loaded CI runners where backstop deadlines fired before background work finished. The views-off eviction test now retries eviction until the root is idle, within the file's 90-second deadline. The startup-step deadline test widens its backstop window from 80 ms to 3 s so executor admission completes first. The blob GC batch test raises a test-only sweep budget from 5 s to 60 s so a loaded runner cannot cut it short. Test-only changes, plus the matching benchmark slice descriptor.