Update integration tests to wait before checking SMS cost data

[klssmith]

What problem does the pull request solve?

The tests which check if the costDetails properties for an SMS notification are not null have been failing at times because those fields are null. Those properties aren't ready immediately, but we can check if they are by checking whether isCostDataReady is true and adding in a wait if the cost data is not ready yet.

Checklist

• [x] I’ve used the pull request template
• [ ] I’ve written unit tests for these changes
• [ ] I’ve updated the documentation in
  • [ ] notifications-tech-docs repository
  • [ ] CHANGELOG.md
• [ ] I’ve bumped the version number (in src/GovukNotify/GovukNotify.csproj)