One member:
Membership: 0
Csize: 1

Five singleton clusters, one merge:
Membership: 1 0 2 0 3
Csize: 2 1 1 1

Six clusters, two merges:
Membership: 1 1 2 0 0 1 1 3 3 0 0 0
Csize: 5 4 1 2

These should fail nicely:
Five singleton clusters, two merges, but second merge refers to singleton which is already merged.
Negative cluster index.
Skip a cluster index.
Too many steps.