[cross_file] Migrate web tests to integration_test

[ditman]

The cross_file package is still using flutter test --platform=chrome (and @TestOn('chrome/vm')) to run its web tests. That is deprecated.

Migrate the web tests of cross_file to an integration test in the example directory, similarly to how it's been done in the flutter/plugins repo.

NOTE: cross_file is only a single, multi-platform package. Ensure that web-only tests only run in web-only environments (with a conditional import, for example).