last weekend I read and very much enjoyed the book "clean code". This great
book contains two chapters on concurrency and recommends to use a tool from
IBM "conTest" to test concurrent code.
This tool fiddles with the byte code of classes under test to increase the
frequency of context switches. Thus conTest tries to make concurrency bugs
more visible, that would normally occur only after many months of production
conTest also can produce a list of classes that are affected by concurrent
code ("Concurrent coverage").
I ran the ZooKeeper tests on my laptop. While the testsuite did never fail on
my machine(TM) before, conTest produced three test failures.
Unfortunatly conTest is not (yet) open source, but free of cost available
 Clean Code: A Handbook of Agile Software Craftsmanship (Robert C. Martin)
Thomas Koch, http://www.koch.ro