Migrate lib_2/isolate tests off unittest

[munificent]

A few of the isolate tests are using the unittest package along with a custom configuration for it used just by those tests. The unittest package was deprecated years ago, well before Dart 2.0. It has not been touched or maintained in ages. These isolate tests are using it:

tests/lib_2/isolate/count_test.dart
tests/lib_2/isolate/cross_isolate_message_test.dart
tests/lib_2/isolate/mandel_isolate_test.dart
tests/lib_2/isolate/message2_test.dart
tests/lib_2/isolate/message_test.dart
tests/lib_2/isolate/mint_maker_test.dart
tests/lib_2/isolate/nested_spawn2_test.dart
tests/lib_2/isolate/nested_spawn_test.dart
tests/lib_2/isolate/raw_port_test.dart
tests/lib_2/isolate/request_reply_test.dart
tests/lib_2/isolate/spawn_function_custom_class_test.dart
tests/lib_2/isolate/spawn_function_test.dart
tests/lib_2/isolate/static_function_test.dart
tests/lib_2/isolate/unresolved_ports_test.dart

They use this helper library which also uses unittest:

tests/lib_2/isolate/remote_unittest_helper.dart

I spent some time trying to migrate these myself, but I don't understand the isolate API well enough to do so. There is no plan to migrate unittest to null safety, which means until these tests are migrated off unittest, there will be no way to migrate these tests to null safety and thus no way to test the corresponding isolate API from null safe code.