Mark test as flaky for all backends

[FlorianHockmann]

We only annotated the test for CQL as flaky, but it's also flaky for other backends. contributor @kptfh already commented on the issue that it's also flaky for the Aerospike backend:
#1498 (comment)

and Natalia recently commented on Discord that the test is flaky for in-memory.

So, we should just move the annotation to the test itself to be not specific for any backend. Since the test is copied to JanusGraphCustomIdTest, we also need the annotation there (and this is also the place where Natalia hit the flaky test).

Issue: #1498 (not fixed by this)

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