In Hadoop 1, testmapredsort is launched from AllTestDriver.java, which was indeed in e.g. hadoop-test-1.2.1.jar. In Hadoop 2, it's now in MapredTestDriver.java, which can be found inside of hadoop/share/hadoop/mapreduce/hadoop-mapreduce-client-jobclient-2.2.0-tests.jar
hadoop jar hadoop/share/hadoop/mapreduce/hadoop-mapreduce-client-jobclient-2.2.0-tests.jar testmapredsort -sortInput /data/unsorted-data -sortOutput /data/sorted-data