Update DelegatingFinder for flutter 3.16.2

[madmini]

When building convenient_test_manager for Windows on master I get the following error message:

> flutter build windows

../convenient_test_dev/lib/src/utils/util.dart(6,7): error G76B49859: The non-abstract class 'DelegatingFinder' is missing implementations for these members: [...\flutter_convenient_test\packages\convenient_test_manager\build\windows\x64\flutter\flutter_assemble.vcxproj]
../convenient_test_dev/lib/src/utils/util.dart(25,21): error G0A556F40: The return type of the method 'DelegatingFinder.evaluate' is 'Iterable<Element>', which does not match the return type, 'FinderResult<Element>', of the overridden method, 'FinderBase.evaluate'. [...\flutter_convenient_test\packages\convenient_test_manager\build\windows\x64\flutter\flutter_assemble.vcxproj]
../convenient_test_dev/lib/src/utils/util.dart(43,10): error GE5CFE876: The method 'DelegatingFinder.toString' has fewer named arguments than those of overridden method 'FinderBase.toString'. [...\flutter_convenient_test\packages\convenient_test_manager\build\windows\x64\flutter\flutter_assemble.vcxproj]
...\Microsoft Visual Studio\2022\Community\MSBuild\Microsoft\VC\v170\Microsoft.CppCommon.targets(254,5): error MSB8066: Custom build for '...\flutter_convenient_test\packages\convenient_test_manager\build\windows\x64\CMakeFiles\ded4cdf977ef776379c919ab64e04105\flutter_windows.dll.rule;...\flutter_convenient_test\packages\convenient_test_manager\build\windows\x64\CMakeFiles\bcf9c5332835a00d3f9f7d34f711e896\flutter_assemble.rule' exited with code 1. [...\flutter_convenient_test\packages\convenient_test_manager\build\windows\x64\flutter\flutter_assemble.vcxproj]
Building Windows application...                                    22,4s
Build process failed.

Looking into DelegatingFinder some methods are incompatible with what I assume were changes introduced in flutter 3.13.0.

I have fixed and extended the class while trying to retain its purpose. With these changes, I am able to build and run the manager as expected, though I have not fully tested it.

[welcome]

Hi! Thanks for opening this pull request! 😄

[fzyzcjy]

LGTM, great job!

[welcome]

Hi! Congrats on merging your first pull request! 🎉

[fzyzcjy]

@all-contributors please add @madmini for code

[allcontributors]

@fzyzcjy

I've put up a pull request to add @madmini! 🎉