This mock was already applied to all tests, because it was done statically
in the TestCurrencyConverter class definition. But it was really not
clear that it's applied everywhere.
Moving this to the setUp() function makes it much clearer.
Also, remove useless redefinition in other tests.
Co-authored-by: Alexis Métaireau <alexis@notmyidea.org>
Currency switching is both simpler and less powerful. This was done primarily for users, to have a clear and logical understanding, but the code is also simpler. The main change is that it is now forbidden to switch a project to "no currency" if bills don't share the same currency.
Also, tests assume that projects are created without currency, as in the web UI.