Update usage of standalone`pub` executable in flutter_tools testing docs (#124898)

Just found this while trying to run the integration test shard of `flutter_tools` locally :)

packages/flutter_tools/test/integration.shard/README.md

@@ -7,7 +7,7 @@ Dart VM and Flutter integration.
 Use this command to run (from the `flutter_tools` directory):
 
 ```shell
-../../bin/cache/dart-sdk/bin/pub run test test/integration.shard
+../../bin/cache/dart-sdk/bin/dart run test test/integration.shard
 ```
 
 You need to have downloaded the Dart SDK in your Flutter clone for this

packages/flutter_tools/test/web.shard/README.md

@@ -7,7 +7,7 @@ Dart web debug services (dwds)  and Flutter integration.
 Use this command to run (from the `flutter_tools` directory):
 
 ```shell
-../../bin/cache/dart-sdk/bin/pub run test test/web.shard
+../../bin/cache/dart-sdk/bin/dart run test test/web.shard
 ```
 
 These tests are expensive to run and do not give meaningful coverage