[Security Solution] Increase parallelism for Defend Workflows tests (#…

…172614)

## Summary

This PR increases the parallelism for the Defend Workflows cypress
tests. I was seeing some long running tests on PRs, this should speed up
PR builds for everyone. All suites run at about 40min or under now.

<img width="1727" alt="image"
src="https://github.com/elastic/kibana/assets/56395104/4673a828-53ef-4e2c-9d65-5d1e99846753">


### Checklist
- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

Co-authored-by: Kibana Machine <[email protected]>

.buildkite/pipelines/pull_request/base.yml

@@ -255,7 +255,7 @@ steps:
       queue: n2-4-virt
     depends_on: build
     timeout_in_minutes: 60
-    parallelism: 10
+    parallelism: 16
     retry:
       automatic:
         - exit_status: '*'