In integration tests where `setBootstrapMasterNodeIndex()` is used in
combination with `autoMinMasterNodes = false` the cluster can start
bootstrapping once the number of nodes set with the
`setBootstrapMasterNodeIndex` have been started but it's not ensured
that all nodes have successfully joined to form the cluster.
This behaviour was introduced with 5db7ed22a0
and in order to ensure that the cluster is properly formed before proceeding
with the integration test, use `ensureStableCluster()` with the
appropriate number of expected nodes.
Fixes: #39220
11fe8cd16f