erp5_test_result: run first test suite with no results

Optimize sorting of list of test suite to be executed by test nodes.
We give priority to test suite with older test results. This allows
to have test node resources in a more fair way.
ERP5Type: add get[Category]ShortTitle accessor

Short title is a property quite widely used, and it happens regularly
to display it in listbox. So such access is very convenient to have
by default.