[#155] Fix test_is_server_ready and add missing global in status checks

[tsmilan]

Summary

This MR fixes the two issues raised in #155

• The is_server_ready PHPUnit data provider now uses a non‑empty hostname for the “503” case so that get_url() returns a valid URL and triggers the status code branch.
• The search_elastic_status_checks() function in lib.php now declares global $CFG; restoring the Elastic check in Site administration > Reports > System status.

Testing

1. Run the Elasticsearch testsuite:
   vendor/bin/phpunit --testsuite=search_elastic_testsuite
   → All cases, including the 503 branch, should pass.
2. Log in as an administrator and navigate to Site administration > Reports > System status.
   → The “Elasticsearch server connection” check appears under status checks.

Closes #155