Introduce example test: passtest

The idea is twofold:
    
 * Show that a test passes simply by not failing, and doing nothing means it can't fail
 * Where appropriate, use this in our own functional tests to reduce the selftest runtime

On my system, the time to run the selftests are:

  * master: 168.25 seconds
  * this branch: 145.2 seconds

A 23 seconds difference (13%) difference.