Test xToMany relationships on backend and e2e side

[Tcharl]

Overview of the feature request

Should be nice to introduce new test that are able to test entities siblings.

Motivation for or Use Case

I sometimes encountered functional regressions on that specific aspect on some jhipster flavour

Related issues or PR

i.e. https://github.com/jhipster/generator-jhipster/pull/18884/files#diff-84e1309b6724f817b2c4b65c1dd87f443c94b13644cd659bf38eaa8ccee87d94

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