Add 'extra' test dep

python-subunit 1.4.3+ requires 'extras' otherwise an error
occures:
"'run --slowest' is not a stestr command."

Switch to using self-signed-certificates-operator as the
self signed functionality has been removed from
tls-certificates-operator charm *1

*1 2cb418cd2e

Change-Id: I8f4da8528cd31b37a8b83e5eab873ebf0e138643

charms/neutron-k8s/test-requirements.txt

@@ -9,3 +9,5 @@ mock
 flake8
 stestr
 ops
+# Subunit 1.4.3+ requires extras
+extras

charms/neutron-k8s/tests/bundles/smoke.yaml

@@ -47,11 +47,10 @@ applications:
     scale: 1
     trust: true
   tls-operator:
-    charm: tls-certificates-operator
+    charm: self-signed-certificates
     channel: edge
     scale: 1
     options:
-      generate-self-signed-certificates: true
       ca-common-name: internal-ca
 
 relations: