concurrent_navigations uses data: URLs, which settle instantly and so never
drove the abort. This test serves a local page with slow subresources plus a
setInterval so the navigation settle loop keeps pumping, then drives it from
four independent CDP connections at once. It aborts the process on the old
single-LocalSet server (SIGTRAP, the V8 heap->isolate() == TryGetCurrent()
fatal) and passes on the thread-per-connection server.
Refs #430.