By extending TestSuite class with Periodicity mixin it is possible to
enable and configure periodicity.

TestSuite.isRestartAllowed used by
ERP5ProjectUnitTestDistributor.createTestResult in order to create new
Test Result for configured periodicity if finished one already exists.

 - [x] remove method isRestartAllowed
 - [x] use method getAlarmDate
 - [x] in ERP5ProjectUnitTestDistributor.py, I would use "alarm_date = test_suite.getAlarmDate()" and then "if alarm_date and alarm_date <= now: ..."
 - [x] set next alarm date

/reviewed-on !479