## Summary

Part of https://github.com/elastic/kibana-team/issues/1503



This PR is mostly about moving tests from
x-pack/test/cases_api_integration and updating related imports

Before:

```
x-pack/test/
     | - cases_api_integration/
```

After: 
```
x-pack/platform/test/
     | - cases_api_integration/
 
x-pack/solutions/security/test/
     | - cases_api_integration/
```

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>