- use mixin class to allow reusing common performance functions
- remove usage of quiet and run_all_test

Reduces the number of meaningless 404 errors.

so that we keep the query string in the redirect url if exists.

Instead of only having possibility to check when inventory will be below a reference_quantity,
allow to check when it will be higher.

This code is useful to run any unit test in a live instance. Until all products tests (like
ERP5/tests/*) are moved as components, this code is useful.

This reverts commit 9240d498.

also stop casting select_language to str.

If ERP5 Jupyter kernel tried to persist an object created from a new-style class, it would crash because the object has no `_p_jar`. This makes the creation of the ObjectWriter fails. This kind of object needs to be handled by (c)Pickle.

Related to bug report https://nexedi.erp5.net/bug_module/20160819-882FA2, created by @kasra.

@Tyagov, please review when possible. 

/reviewed-on !162

erp5_item: the purpose of getTrackingList or getCurrentTrackingList is to avoid passing simulation states by hand

Some tests where explicitely checking that transformation is not set in case
several are found. It is indeed better to ask user decision in such case.

mrp: a production order has a specialise link for business template, so we have to look for transformations

erp5_travel_expense: Add excel download button as an example and rename megapixel javascript name to travel expense dependent one to avoid confliction.

because objects will be lost.

At same time, simplify code a bit :

- Just use _migration_in_progress attribute directly instead of a variable.
- Set a default value on class, so that we do not have to getattr.
- Do not use  aq_base, this is useless for attributes starting with _

With previous algorithm, work was given to additional test nodes only when:
- we were previously below the needed capacity
- when another test node was dying

Now, as soon as a new test node is added, we move work of overloaded test nodes to
idle test nodes. We try to move only test suite using many test nodes to avoid having to
wait for building time.

This allows to have better distribution of the work with the idea to have more quickly test results.
This will avoid cases where we have several testnodes assigned to no work at all.

Finally, fixed distribution algorithm to avoid some unfair cases where a test suite might
have more test node than another while they both ask for the same number of test nodes.