protected AbstractStickyAssignor.MemberData memberData(Subscription subscription) {
        return assignor.memberData(subscription);
    }
Verifies that the given assignment is valid with respect to the given subscriptions
Validity requirements:
- each consumer is subscribed to topics of all partitions assigned to it, and
- each partition is assigned to no more than one consumer
Balance requirements:
- the assignment is fully balanced (the numbers of topic partitions assigned to consumers differ by at most one), or
- there is no topic partition that can be moved from one consumer to another with 2+ fewer topic partitions

@param subscriptions: topic subscriptions of each consumer
@param assignments: given assignment for balance check
@param partitionsPerTopic: number of partitions per topic
clients/src/test/java/org/apache/kafka/clients/consumer/internals/AbstractStickyAssignorTest.java