[tool] Add tests for FakeProcessManager (#104456)

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