[Alerting] Migrate last pieces of legacy fixture code

[gmmorris]

We couldn't migrate the following fixtures:

1. Slack - as it needs custom error responses. Issues submitted to Platform: kibanaResponseFactory should support entirely custom responses #65045
2. Webhook - as it uses a custom authorisation type which Platform doesn't expose anymore.

   kibana/x-pack/test/alerting_api_integration/common/fixtures/plugins/actions_simulators_legacy/webhook_simulation.ts

   Line 18 in 94127d8

   server.auth.scheme('identifyCredentialsIfPresent', function identifyCredentialsIfPresent(

Regarding Webhook we should look into whether this custom auth is really needed and if so - submit an issue to Platform. Hopefully we can just change how we test this.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[pmuellr]

Slack - as it needs custom error responses. Issues submitted to Platform: #65045

That issue was closed with "The recommended solution for this particular use-case is to switch to another HTTP server", so I think we'll have a bit more work to do here than originally thought. Basically, we need to pull in some other HTTP server or use node http APIs bare (painful) instead of the in the FTS we currently run them with.

Webhook - as it uses a custom authorisation type which Platform doesn't expose anymore.

I don't know much about this one, but if we do end up needing a new server for Slack, then probably the easiest way to deal with Webhook is to also use the separate server for that as well.