fix(test): converted some MockWebServer to long-running instances

[paullatzelsperger]

WHAT

this PR attempts to fix some of the flaky e2e tests we saw in the past.
The problem there was that the MockWebServer could not be started, due to an "Address already in use" error.

We don't exactly know where this is coming from, so this is really more of an "exploratory" PR

It should be noted that this goes somewhat against the recommended use of the MockWebServer, which requires it to be re-instantiated on every test.

WHY

Flaky e2e tests

FURTHER NOTES

other solution ideas are

• switching to another mockwebserver. In EDC we use the Netty MockServer, which seems to not have these problems.
• adding a RetryPolicy to the instantiation and starting of the mock webserver: this would guard against transient errors such as timing issues
• use randomized ports for every instance: would probably the cleanest solution, but might be difficult to implement, since all in-mem runtimes are instantiated only once, and would need to be re-instantiated.

Closes # <-- insert Issue number if one exists

[wolf4ood]

🤞

[sonarcloud]

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