## Summary

Part of elastic/kibana-team#1503

This PR is mostly about moving platform tests from x-pack/test/

Before:

```
x-pack/test/
     | - ftr_apis/
     | - load/
     | - plugin_api_perf/
     | - fleet_packages/
     | - fleet_tasks/
     | - scalability/
     | - task_manager_claimer_update_by_query/
```

After:
```
x-pack/platform/test/
     | - ftr_apis/
     | - load/
     | - plugin_api_perf/
     | - fleet_packages/
     | - fleet_tasks/
     | - scalability/
     | - task_manager_claimer_update_by_query/

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
(cherry picked from commit 58b8c7a)

# Conflicts:
#	.buildkite/ftr_platform_stateful_configs.yml
#	.github/CODEOWNERS
#	x-pack/platform/test/fleet_tasks/config.ts
#	x-pack/platform/test/fleet_tasks/ftr_provider_context.d.ts
#	x-pack/platform/test/fleet_tasks/helpers.ts
#	x-pack/platform/test/fleet_tasks/tests/automatic_upgrades.ts
#	x-pack/platform/test/fleet_tasks/tests/index.ts
#	x-pack/test/fleet_tasks/ftr_provider_context.d.ts
#	x-pack/test/fleet_tasks/tests/index.ts
#	x-pack/test/functional_enterprise_search/ftr_provider_context.d.ts
#	x-pack/test/tsconfig.json
#	x-pack/test/upgrade/apps/logs/index.ts

…fix'