Closes https://github.com/flutter/flutter/issues/20949.
Signals (such as SIGTERM or SIGKILL) end up flowing through
`exitWithHooks`, which in turn, after running hooks, call `exit().` That
means, as a result, any `try { } finally { }` guarded execution may
_not_ run, which happens to also be how `flutter_tester` instances are
cleaned up if they have not terminated.
This PR adds in-progress `flutter_tester` runs (or any platform
`flutter_platform` supports) to the shutdown hooks, guaranteeing that
the finalizers (which in turn, kill the process) are _always_ executed
as long as either the test completes, _or_ `exitWithHooks` is called.
The existing integration tests (`integration.shard/test_test.dart`)
still pass as well.
c5379557b3