Make cleanup of flutter processes in tests more reliable (#19307)

* Make cleanup of flutter processes in tests more reliable

* Fix quit signals

I confused SigInt&SigTerm for SigTerm&SigKill when I first did this. SigTerm can be blocked and doesn't guarantee the process will be terminated.

* Don't use deprecated constants

* Fix typo

* Add some additional info to debug buffer

* Fix return types on Futures