Rename test folder

### Motivation
The final task left to close #275 is to rename the folder `test` to `tests`. This is nice, given that `test` is a module of the Python Standard Library.

### Changelog
* Move test files from `./test` to `./tests`.
* Refactor tests of `db_store` to use temporary files.
* Test operations on encrypted as well as on unencrypted DB, as to detect authentication errors.