A green bar for a good feeling! Thanks to the help and support of Dave Carver who finally fixed all failing JUnit tests, the build (actually the tests) is green. Download the latest XML Security Tools release from http://build.eclipse.org/webtools/committers. Attention: this is still an integration build, so use it at your own risk!
Reason for the failing JUnit tests were some differences between my local system and the build environment. What I know now is that the build system requires a different resource file loading. In other words: the complete path to a file, which is not required when running a JUnit test plug-in in a local workspace.