When the engine dies unexpectedly during test execution, we have to
terminate any tests running in that engine. Previously, they would just
hang. For some reason that I was never able to satisfactorily explain,
the WebSocket doesn't die in a way I can detect in this case. So
instead, we hand in a future that we only complete when we detect the
server subprocess ends.
This command uses package:test to run Dart tests with sky_shell. For this to
work, we need https://github.com/dart-lang/test/tree/hacky-loader-hook to land.
We're also not smart enough to find sky_shell ourselves yet. Instead, we take
the path as input using an environment variable. Eventually, we'll be able to
get the sky_shell executable from package:sky_engine, but we don't yet ship
that executable.
