Add testing

[marcoow]

I have basic DB access and CRUD requests working. Next I'd like to test such that:

• I can test individual endpoints.
• For each test, I can set up the scenario directly interacting with the database ("it responds with the correct JSON when x users are in the database", "it inserts a new user into the database", etc.).
• Tests run in their own database transaction so there's no state leaked across tests (and they could run in parallel using the same database instance without interfering with each other).

[marcoow]

It seems like crates.io is doing something like what I want as well:

https://github.com/rust-lang/crates.io/blob/ea7cc3c3d44faa8055c9f6774b150ae5788276aa/src/tests/categories.rs#L40-L46

I'm not sure where the transaction is rolled back but maybe that's not necessary anyway as that might be automatic once the connection is closed…

[marcoow]

There was a talk on the topic at EuroRust as well: https://www.youtube.com/watch?v=_VB1fLLtZfQ

[marcoow]

The talk suggest using database closing instead of transactions which seems interesting.

[marcoow]

done