[tool] Consistent FakeProcessManager.run/runSync

[cbracken]

FakeProcessManager is a test-oriented implementation of ProcessManager
that simulates launching processes and returning ProcessResult objects
whose exitCode, stdout, stderr can be used to write platform-portable,
hermetic tests that don't rely on actually launching processes from
executables on disk. Its run and runSync methods provide asynchronous and
synchronous variants of this functionality.

Previously, the behaviour of run and runSync were inconsistent with
regards to the treatment of the stdoutEncoding (similarly,
stderrEncoding) parameters:

run:

• if the encoding was null, ProcessResult.stdout was returned as a
  String in UTF-8 encoding. This was incorrect. The behaviour as
  specified in ProcessResult.stdout is that in this case, a raw
  List<int> should be returned.
• If the encoding was unspecified, ProcessResult.stdout was returned as
  a String in the io.systemEncoding encoding. This was correct.
• If the encoding was non-null, ProcessResult.stdout was returned as a
  String in the specified encoding. This was correct.

runSync:

• if the encoding was null, ProcessResult.stdout was returned as a
  List<int> in UTF-8 encoding. This was incorrect. The behaviour as
  specified in ProcessResult.stdout is that in this case, a raw
  List<int> should be returned.
• If the encoding was unspecified, ProcessResult.stdout was returned as
  List<int> in UTF-8 encoding. This was incorrect. The behaviour as
  specified in ProcessResult.stdout is that in this case, a String a
  String in the io.systemEncoding encoding should be returned.
• if the encoding was non-null, ProcessResult.stdout was returned as a
  String in unknown (but probably UTF-8) encoding. This was incorrect.
  The behaviour as specified in ProcessResult.stdout is that in this
  case, a String in the specified encoding should be returned.

_FakeProcess, from which we obtain the fake stdout and stderr values now
holds these fields as raw List<int> of bytes rather than as Strings. It
is up to the user to supply values that can be decoded with the encoding
passed to run/runAsync.

run and runAsync have been updated to set stdout (likewise, stderr) as
specified in the ProcessResult documentation.

This is pre-factoring for #102451, in which the tool throws an exception
when processing the JSON output from stdout of the vswhere.exe tool,
whose output was found to include the U+FFFD Unicode replacement
character during UTF-8 decoding, which triggers a toolExit exception
when decoded using our Utf8Decoder configured with reportErrors =
true. Because FakeProcessManager.runAsync did not previously invoke
utf8.decode on its output (behaviour which differs from the non-fake
implementation), it was impossible to write tests to verify the fix.

Ref: https://api.flutter.dev/flutter/dart-io/ProcessResult/stdout.html

Issue: #102451

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[jonahwilliams]

LGTM