In another change (#37646), I want to test that a test fails and
prints expected output.  I didn't see an existing way to do that, so
I modified `_runFlutterTest` and `runCommand` to allow capturing the
output.  Currently capturing and printing output are mutually
exclusive since we don't need both.

Some awkward bits:
* There already exists a `runAndGetStdout` function that is very
  similar to `runCommand`, and this change makes the conceptual
  distinction more confusing.

* `runFlutterTest` has multiple code paths for different
  configurations.  I don't understand what the different paths are
  for, and I added output checking only along one of them.