pin test to 1.31.0 to match the Flutter SDK's test_api

The SDK's flutter_test pins test_api 0.7.11, but test 1.30.0 requires
0.7.10, so dependency resolution failed and make run/test aborted.
1.31.0 is the release whose test_api dep is exactly 0.7.11; 1.31.1 wants
0.7.12 and would re-conflict. Pulls in the transitive meta 1.18.0 and a
Dart >=3.10 lower bound.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pubspec.yaml

@@ -62,7 +62,7 @@ dev_dependencies:
   integration_test:
     sdk: flutter
 
-  test: 1.30.0
+  test: 1.31.0
   mocktail: 1.0.4
   alchemist: 0.12.1