[Maintenance] improve testing utils and add delete-user-file test

[tcrasset]

Adding integration tests is a pre-requisite to #420, as refactoring might break some stuff without us knowing, and manually doing the actions on the front-end is a pain.

I wanted to create a small PR that shows the value of adding integration tests.
I kept it small on purpose so that I wouldn't have done this for nothing if it gets rejected.

My plan is that this gets accepted, and I/we start adding integration test on all/major endpoints, so that we can more readily refactor the backend to be inline with #420.

[tcrasset]

The /delete-user-file req.body was not being parsed as json by default, which necessitates the middleware to be added.

I manually tested that the /sync endpoint still works by launching the frontend add doing some syncable stuff, and watching the requests/responses.
It would be nice if there were tests for this function, which is my next plan.

[tcrasset]

Took your comments into account.

Also, during some more integration testing for /sync, I noticed that I actually needed the middleware to be express.raw for the /sync endpoint to work during integration tests. I would wager that this is specific to jest or supertest, as I had no issues without it when using the frontend. Same for the delete-user-file endpoint, worked prefectly using curl without specifying the express.json() middleware, but during integration testing it complained the body was empty, as specified in one of my previous comments.

Am not knowledgeable enough in javascript testing to know why that might be.

[tcrasset]

Follow up PR is here: #423