Add the necessary API so that we can implement new tests easily as new
standalone classes.
In Condition::DSL:
* Add #tests accessor;
* Modify constructor so that it accepts the env;
* Implement .define_test(keyword, class) method.
Instead of interrupting task evaluation when condition is not met,
allow the whole task to be evaluated (including condition and
evaluation) so that the interpreter will get all tasks actions (whether
condition is met or not) and be able to query the condition.
* Modify Interpreter#process_task: test if task condition is met before
applying the actions;
* Implement condition handling in Task and Task::DSL;
* Implement Condition and Condition::DSL (useless as they are, but
needed to implement later test keywords as part of the condition DSL.
