CI: replace Sonobuoy with Hydrophone for conformance smoke test (#13680)

Reuse the existing Hydrophone installation and preserve the single test selected by Sonobuoy Quick mode. Rename the opt-in variable to conformance_enabled and update the Elastx testcase.

tests/testcases/tests.yml

@@ -40,12 +40,4 @@
       import_tasks: 100_check-k8s-conformance.yml
       delegate_to: "{{ groups['kube_control_plane'][0] }}"
       run_once: true
-      when:
-        - sonobuoy_enabled is defined
-        - sonobuoy_enabled
-      vars:
-        sonobuoy_version: 0.57.3
-        sonobuoy_arch: amd64
-        sonobuoy_parallel: 30
-        sonobuoy_path: /usr/local/bin/sonobuoy
-        sonobuoy_mode: Quick
+      when: conformance_enabled | default(false)