Adds a bit more clarifying documentation to the implementation of the
outputFollowsExit case, and adds tests that verify the behaviour of
stderr, stdout of processes launched via FakeProcessManager.
Specifically:
* Verifies that stderr, stdout are not emitted immediately after process
exit if outputFollowsExit is true. They must be emitted at least one
turn through the event loop later.
* Verifies that ProcessResult.stderr, stdout have the type documented
according to the encoding passted to Process.run/runSync:
* List<int> if null is passed as the encoding.
* String (in the default system encoding) if no encoding is specified.
* String (in the specified encoding) if an encoding is specified.
This is additional testing relating to refactoring landed in:
https://github.com/flutter/flutter/pull/103947
Issue: https://github.com/flutter/flutter/issues/102451
Because this class has some subtle behaviour with regards to control of
exit timing and when and how it streams data to stderr and stdout, it's
worth adding unit tests for this class directly, as well as (in a
followup patch) for FakeProcessManager.
This is additional testing relating to refactoring landed in:
https://github.com/flutter/flutter/pull/103947
Issue: https://github.com/flutter/flutter/issues/102451
