stack/erp5: expose random_activity_priority argument of testrunner

By setting this in the test suite configuration, we can force ERP5 tests
to use random priorities for activity processing.

The intended usage is to set this to an empty string, in which case
ERP5's runUnitTest chooses a random value for the seed and prints the
chosen value. It may also be possible to use a value, to reproduce a
previous run.