"Normalize" tests

[Naapperas]

Right now there are tests that manage (create/delete) the resources they need and there are others that use what is available from prior tests. This can cause conflicts that should not happen, since the tests, or test groups (mainly tests inside a describe) should be independent from each other, and not have any influence on tests that appear after.

So the test suites should be refactored in order to have each test/test group use only the resources they need, creating them and deleting them, without leaving "residues" for other tests.

[Naapperas]

I had problems regarding this when working on #168: in the edit offer tests, I had to change the date of creation of the offers because the dates previously used were causing conflicts with another test later on

[BrunoRosendo]

We need to prioritize this issue very soon.
We should also split the test files, maybe by route. The test files are way too big right now. Small discussion here

[Process-ing]

While working on issue #261, I noticed that some offer publish dates in the offer and company tests were outdated (see more details in the issue). It's most likely that other dates are also outdated, leading to unwanted behavior in the tests, so those should be checked.

[BrunoRosendo]

We can consider separating tests into different files and also have some listener that purges all the database's information. This could happen in each test or group. This is done in https://github.com/NIAEFEUP/website-niaefeup-backend but it's a totally different tech