This revert a wrong change in commit 8995b645

Move active context creation, active callable creation, packet length test
out of loop.

That change makes no sense latency-wise, iteration will anyway happen
inside "list()". Also, it potentially degrades memory usage.

use key press instead of click on button save to avoid permission denied failure with selenium and firefox

Reduce differences between similar tests.
Remove bit-rot comments.
Simplify code (avoid redundant lookups, ...).
Also, make easier to subclass and use for a different category tree.

Still, don't fail depending on day of week.

Also, update exception raising style.

Those are single-valued, so just check consistency internally and return
a single value.
Changelog and some cleanups by Vincent Pelletier <vincent@nexedi.com>.

It is known to cause table locks when no value is provided for such
column. We now always generate uids in ERP5, so this setting is useless.

-user with correct permissions should be able to set start or stop date. Add proxy role manager in scripts.

See testTradeModelLine for new use of taxes/discounts.

This avoid random permission errors (like if another user run the
same test)

This is a followup to commit 8995b645
("always start transactions with begin() and make tic() commit automatically"):
there is now no need to use self.stepTic() instead of self.tic()