The newly added testcase is based on the already existing
`test_alter_dropped_tablets_keyspace`.
A new error injection is created, which stops the ALTER execution just
before the changes are submitted to RAFT. In the meantime, a new schema
change is performed using the 2nd node in the cluster, thus causing the
1st node to retry the ALTER statement.
(cherry picked from commit 522bede8ec)
a61ab7d02e