Add e2e tests for interaction through Argo CD API

[jannfis]

Is your task related to a problem? Please describe.

Our end-to-end tests create apps to be synced in a declarative way, i.e. create the Application resources directly on the clusters. However, there are subtle differences between creating the apps in a declarative way and creating them through Argo CD.

For example, we broke something (#412) without the end-to-end test noticing that.

Describe the solution you'd like

The end-to-end tests should have one or more test cases creating and deleting applications through the Argo CD API. There are already some fixtures in place to interact through the Argo CD API, they might have to be enhanced. We do not need a full sync-to-agent cycle. For now, we should test whether we're able to:

1. Create an Application
2. List and get more details about the Application
3. Delete the Application

Describe alternatives you've considered

Additional context