[tool] Accept plutil diagnostics from stderr in plist_parser_test (macOS 26) (#193292)

Fixes https://github.com/flutter/flutter/issues/193159

## What's failing

Two tests in `plist_parser_test.dart` fail on macOS 26 (first seen on
`staging.shadow/Mac tool_integration_tests_1` at Mac-26.6.2 / Xcode
17C52,
https://ci.chromium.org/b/8670252478857878881):

- `PlistParser.getValueFromFile<String> returns null for a malformed
plist file`
- `PlistParser.replaceKey returns false for a malformed plist file`

Both tests still get the right result (`null` / `false`, and `plutil`
still
exits 1). What changed is where `plutil` prints its error:

- Before macOS 26, `plutil` printed `<path>: Property List error: … /
JSON error: …`
  on **stdout**.
- On macOS 26, `plutil` was rewritten (Apple open-sourced the new
version in
swift-corelibs-foundation, `Sources/plutil/PLUContext.swift`) and the
same
  message now goes to **stderr**.

`ProcessUtils.runSync` logs a failed command's stdout with `printStatus`
and
its stderr with `printError`. So on macOS 26 `logger.statusText` is
empty and
the diagnostic shows up in `logger.errorText` instead, ahead of the
`ProcessException:` line. The tests only looked at `statusText`.

#172913 ran into the same plutil change last year in
`xcode_backend.dart`.

## The fix

Test-only change. `PlistParser` itself is fine.

- Look for the plutil diagnostic in `statusText + errorText` rather than
`statusText` alone, so the test passes whichever stream plutil uses. The
full message is still asserted, so a change in the wording would still
be
  caught.
- Check that `errorText` ends with the `ProcessException` block instead
of
  equalling it, since on macOS 26 the plutil message now comes first.

## How I verified it

- Ran the tests locally.
- led run of `Mac tool_integration_tests_1` (subshard `1_10`, the one
that
contains `plist_parser_test.dart`) on a Mac-26.6.2 bot (`mac-340-g140`,
  the same bot as the original failure) at this PR's head:
https://ci.chromium.org/b/8669753954317678657 — green, both tests pass.

## Pre-launch Checklist

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

[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md