Fix 428e9dad.

i.e support mocking of slaplib in tests.
Also document about weird behavior of slaplib.

Mocking slaplib is good, duplicating all its codebase is wrong.
Should use either a simple mockup, or the real slaplib.

It seems it confuses newcomers for now.

This enables the right acl_user, instead only shadow one.

  setUpOnce is executed on save but afterSetUp is not.

  bootstrapSite should be invoked once per Test Case run instead a single
  time on save phase.

This prevent increasing the memory usage due to huge workflow history.

distribution check: added a few tests