ci(e2e): Changed UNSTABLE state to be considered as a failure

UNSTABLE state means some tests have failed during the retries but also in the last retry so, it contemplates real failures too.

ci/Jenkinsfile.e2e

@@ -143,8 +143,7 @@ pipeline {
             testSuite: "${WORKSPACE}/test/ui-test/testSuites/*",
           ])
           print("Squish run result: ${result}")
-          /* Ignore UNSTABLE caused by retried tests. */
-          if (!['SUCCESS', 'UNSTABLE'].contains(result)) {
+          if (!['SUCCESS'].contains(result)) {
             throw new Exception('Squish run failed!')
           }
         } }