public class SomeBatch implements Database.Batchable<SObject> {
  @TestVisible String query = 'SELECT Id FROM User WHERE ...';
  public Database.QueryLocator start(Database.BatchableContext context) {
    return Database.getQueryLocator(query);
  }
  ...

SomeBatch b = new SomeBatch();
b.query = 'SELECT Id FROM User WHERE ... LIMIT 200';
Test.startTest();
Database.executeBatch(b);
Test.stopTest();
// verify results