caddy-frontend/test: Request instance with slaves

This test just update slave dict of the class, so it is needed to request.

software/caddy-frontend/test/test.py

@@ -6437,6 +6437,7 @@ class TestSlaveSlapOSMasterCertificateCompatibilityUpdate(
       'apache-key': key_pem,
 
     })
+    self.requestDefaultInstance()
     self.slap.waitForInstance()
     self.runKedifaUpdater()