Fix unit test warning with many-to-many

[hdurix]

Overview of the issue

In update management spec, when the entity has got a many-to-many relationship and is owner-side, there is a Jest error:

<select multiple v-model="post.tags"> expects an Array value for its binding, but got Undefined

Reproduce the error

It can be reproduce with https://github.com/jhipster/jdl-samples/blob/master/microservices-blog-store.jh#L52-L83.

Suggest a Fix

In the test, many-to-many should be initialized, like this:

const entity = { id: 123, tags: [] };

For that, we should iterate on the many-to-many relationships in this test and this test like we do for the implementation.