private ProcedureExecutor<MasterProcedureEnv> getMasterProcedureExecutor() {
    return UTIL.getHBaseCluster().getMaster().getMasterProcedureExecutor();
  }
This tests two concurrent region merges
hbase-server/src/test/java/org/apache/hadoop/hbase/master/assignment/TestMergeTableRegionsProcedure.java