[flutter_tools] Add tests for negative lookahead regex in test runner and batch entrypoints (#191438)

## Description

Fixes https://github.com/flutter/flutter/issues/84270

The root cause of #84270 (delayed variable expansion in batch scripts
stripping `!` from CLI arguments like `^(?!Golden).+`) was previously
addressed in #106861, which removed `SETLOCAL ENABLEDELAYEDEXPANSION`
from `bin/flutter.bat`, `bin/dart.bat`, and `bin/internal/shared.bat`.

However, #84270 was left open without dedicated regression tests. This
PR:
1. Adds a hermetic unit test in `test_test.dart` verifying that
`TestCommand` preserves and forwards negative lookahead regexes
(`^(?!Golden).+`) to `FlutterTestRunner`.
2. Adds a case-insensitive static test in `script_test.dart` asserting
that Windows entrypoint batch scripts do not enable delayed expansion.
3. Unskips and modernizes the integration test in
`variable_expansion_windows_test.dart` verifying that `bin/dart.bat`
preserves lookahead regexes on Windows.

## 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.
- [ ] 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] All existing and new tests are passing.

<!-- Links -->
[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/