Allow using a file to exclude certain tests from build

It would be nice to be able to exclude certain tests when running builds. For example, when a test is "known flaky", you may want to exclude it from the main Hudson job, but not actually disable it in the codebase (so that it still runs as part of another Hudson job, for example).

By default all tests are run. However if a special command line parameter is added the specified file will be used to exclude certain tests:

ant -Dtest.exclude.file=flakeytests

The contents of flakeytests could be ant path regexes:

**/TestDiskFailoverBehavior.java
**/TestDiskFailover*.java

Strikingly similar to https://issues.apache.org/jira/browse/HADOOP-7167