Add custom_injectors to test code path

* Unit tests do not create CredentialType records for Credential
  plugins. Instead, they explicitly instantiate CredentialType(s) for
  Credential plugins. They rely on CredentialType.defaults[key] to do
  so. This change makes sure custom_injectors get bolted onto the
  created CredentialType.

awx/main/models/credential.py

@@ -710,7 +710,9 @@ class ManagedCredentialType(SimpleNamespace):
         )
 
     def create(self):
-        return CredentialType(**self.get_creation_params())
+        res = CredentialType(**self.get_creation_params())
+        res.custom_injectors = self.custom_injectors
+        return res
 
 
 class CredentialInputSource(PrimordialModel):