pkg/csslib: use pkg/test and allow latest version of logging pkg

test/debug_test.dart

 library debug;
 
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 import 'testing.dart';
 
 void main() {
   test("excercise debug", () {
     var style = parseCss(_input);
 
     var debugValue = style.toDebugString();
     expect(debugValue, isNotNull);
 
     var style2 = style.clone();
 
     expect(style2.toDebugString(), debugValue);
   });
 }
 
 const String _input = r'''
 .foo {
 background-color: #191919;
 width: 10PX;
 height: 22mM !important;
 border-width: 20cm;
 margin-width: 33%;
 border-height: 30EM;
 width: .6in;
 length: 1.2in;
 -web-stuff: -10Px;
 }''';