No test in SimpleAssignorTest covers the case where a member's current
assignment contains a topic that the share group no longer subscribes
to. In that case,
SimpleHomogeneousAssignmentBuilder#revokeUnassignablePartitions removes
the topic from the member's assignment.

This adds a homogeneous test with two members that each hold one
partition of topic 1 and one partition of topic 2, after the group's
subscription changes to topic 1 only. It verifies that topic 2 is
revoked, that each member keeps its partition of topic 1, and that the
remaining partitions of topic 1 are spread so that each member ends up
with two.

Topic 1 has four partitions so that revoking topic 2 leaves each member
below its target, which also checks that the freed capacity is filled
from the unassigned partitions of topic 1.