Check support for using the return statement in a task.
* That it is possible to exit form a task using the `return` statement
without affecting other concurrently running instances of the same task
* That it is possible to use return in a named block in a task
* That using a return value in a task results in a elaboration error
* Returning from inside a parallel block in a task results in a
elaboration error
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>