documentations on how to write unit testing and e2s with mongoose

[getspooky]

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[ ] Regression 
[ ] Bug report
[ ] Feature request
[x] Documentation issue or request (new chapter/page)
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Current behavior

Currently, the documentation does not contain details on how to write unit tests and e2e tests with mongoose and nestjs.

Expected behavior

NestJS docs should at least have a section Database Testing with examples to get developers started on how they can run good tests