[Security Solution] Fix Endpoint tests when prepackaged with a dev endpoint package (#147052)

## Summary

Change our query to use `prerelease` to correctly find dev endpoint
packages in tests


### 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

fleet_packages.json

@@ -29,7 +29,7 @@
   },
   {
     "name": "endpoint",
-    "version": "8.6.1"
+    "version": "8.7.0-next"
   },
   {
     "name": "fleet_server",

x-pack/test/security_solution_endpoint/services/endpoint_policy.ts

@@ -33,7 +33,7 @@ const INGEST_API_PACKAGE_POLICIES = `${INGEST_API_ROOT}/package_policies`;
 const INGEST_API_PACKAGE_POLICIES_DELETE = `${INGEST_API_PACKAGE_POLICIES}/delete`;
 const INGEST_API_EPM_PACKAGES = `${INGEST_API_ROOT}/epm/packages`;
 
-const SECURITY_PACKAGES_ROUTE = `${INGEST_API_EPM_PACKAGES}?category=security&experimental=true`;
+const SECURITY_PACKAGES_ROUTE = `${INGEST_API_EPM_PACKAGES}?category=security&prerelease=true`;
 
 /**
  * Holds information about the test resources created to support an Endpoint Policy