private void requestSplitRegion(final HRegionServer rsServer, final Region region,
    final byte[] midKey) throws IOException {
    long procId = cluster.getMaster().splitRegion(region.getRegionInfo(), midKey, 0, 0);
    // wait for the split to complete or get interrupted. If the split completes successfully,
    // the procedure will return true; if the split fails, the procedure would throw exception.
    ProcedureTestingUtility.waitProcedure(cluster.getMaster().getMasterProcedureExecutor(), procId);
  }
The below tests are testing split region against a running cluster
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestSplitTransactionOnCluster.java