Fix and clean up url launcher tests

[jpnurmi]

The platform interface has changed so that all launch options are
bundled into an object. We don't really care about the exact options,
just using the defaults, so it can be "any" instead of trying to match
the values.

[jpnurmi]

This fixes the test failure in #946:

00:37 +74 -1: /home/runner/work/ubuntu-desktop-installer/ubuntu-desktop-installer/packages/ubuntu_wizard/test/url_launcher_test.dart: can launch [E]                                                   
  type 'Null' is not a subtype of type 'Future<bool>'
  package:url_launcher_platform_interface/src/url_launcher_platform.dart 72:16  MockUrlLauncherPlatform.launchUrl
  package:url_launcher/src/url_launcher_string.dart 33:45                       launchUrlString
  package:ubuntu_wizard/src/utils/url_launcher.dart 18:12                       UrlLauncher.launchUrl
00:37 +74 -1: /home/runner/work/ubuntu-desktop-installer/ubuntu-desktop-installer/packages/ubuntu_wizard/test/url_launcher_test.dart: cannot launch                                                    
00:37 +75 -1: /home/runner/work/ubuntu-desktop-installer/ubuntu-desktop-installer/packages/ubuntu_wizard/test/url_launcher_test.dart: cannot launch                                                    
00:37 +75 -1: /home/runner/work/ubuntu-desktop-installer/ubuntu-desktop-installer/packages/ubuntu_wizard/test/url_launcher_test.dart: launch fail                                                      
00:37 +75 -2: /home/runner/work/ubuntu-desktop-installer/ubuntu-desktop-installer/packages/ubuntu_wizard/test/url_launcher_test.dart: launch fail [E]                                                  
  type 'Null' is not a subtype of type 'Future<bool>'
  package:url_launcher_platform_interface/src/url_launcher_platform.dart 72:16  MockUrlLauncherPlatform.launchUrl
  package:url_launcher/src/url_launcher_string.dart 33:45                       launchUrlString
  package:ubuntu_wizard/src/utils/url_launcher.dart 18:12                       UrlLauncher.launchUrl
00:37 +75 -2: /home/runner/work/ubuntu-desktop-installer/ubuntu-desktop-installer/packages/ubuntu_wizard/test/url_launcher_test.dart: service                                                          
00:37 +75 -3: /home/runner/work/ubuntu-desktop-installer/ubuntu-desktop-installer/packages/ubuntu_wizard/test/url_launcher_test.dart: service [E]                                                      
  type 'Null' is not a subtype of type 'Future<bool>'
  package:url_launcher_platform_interface/src/url_launcher_platform.dart 72:16  MockUrlLauncherPlatform.launchUrl
  package:url_launcher/src/url_launcher_string.dart 33:45                       launchUrlString
  package:ubuntu_wizard/src/utils/url_launcher.dart 18:12                       UrlLauncher.launchUrl

[CarlosNihelton]

Great! Cleanups are always welcome!