Development: Refactor LinkPreviewIntegrationTest

[reschandreas]

Describe the bug

Since some time, the tests for the LinkPreviewIntegrationTest fail constantly on GitHub, I fixed them temporarily in #8037 so that all our tests get executed, but this class needs refactoring.

Currently, the test will actually call foreign domains which are not under our control, the PR #8037 changes that, but in my understanding, this should be mocked to work without an internet connection to qualify as an integration test.

To Reproduce

Run the server tests.

Expected behavior

Actual integration tests with mocks are written. The tests must pass without an internet connection and not DOS another domain.

Screenshots

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Which version of Artemis are you seeing the problem on?

on develop

What browsers are you seeing the problem on?

Other (specify in "Additional context")

Additional context

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Relevant log output

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