Use common's functions where possible

[julienw]

In #47 there is a new common.js file whose goal is to build a toolkit for the most current operations. The goal is to use it for all needed set-up in the tests.

Be careful: do not use it for the actual tested behavior. For example, do not use common's loginAsAdmin if you're actually testing the admin login. Use it only when it's not the test's subject and it's merely a setup step to the test.