[analyzer] tests on Windows have different path style for different files

[asashour]

For example, on running a data driven test, while writing the path of files created in

https://github.com/dart-lang/sdk/blob/fba97eb77f1218025e73134a9cbfe94f49dcb98d/pkg/analysis_server/test/abstract_single_unit.dart#L56

The output is:

path /home/test/.dart_tool/package_config.json
path C:\home\test\analysis_options.yaml
path /home/p/lib/lib.dart
path /home/test/.dart_tool/package_config.json
path /home/p/lib/expect.dart
path /home/p/lib/export.dart
path /home/p/lib/fix_data.yaml
path C:\home\test\lib\test.dart

I believe they all should be either in Windows or in Unix, but not a mix, as this is somewhat confusing and could be a potential source of errors.

Dart SDK version: 2.19.0-215.0.dev (dev) (Sun Sep 18 20:34:31 2022 -0700) on "windows_x64"