The displaytag test suite would not run in my sandbox because its path contained a space, namely the space in Windoze's "c:\Documents and Settings" folder. TestAll.findAndStoreTestClasses would blow up with a null-pointer exception, blocking all unit-test execution and generating the following report.
Testsuite: org.displaytag.test.TestAll
Tests run: 1, Failures: 0, Errors: 1, Time elapsed: 0 sec

Null Test: Caused an ERROR
null
java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
Caused by: java.lang.NullPointerException
at org.displaytag.test.ClassFinder.findAndStoreTestClasses(TestAll.java:180)
at org.displaytag.test.ClassFinder.<init>(TestAll.java:156)
at org.displaytag.test.TestAll.suite(TestAll.java:104)
... 7 more
See http://jira.codehaus.org/browse/DISPL-243#action_46601

The problem is the encoded space in the URL used to load the test resources, such that TestAll would attempt to look for test files in "c:\Documents%20and%20Settings"/blah/blah/blah instead of "c:\Documents and Settings\blah\blah\blah.
Attached you'll find a patch that fixes this bug by decoding the URL involved before attempting to find files in the local system.