DaCapo xalan requires the VM to use a specific version of xalan,
which is not the same version found in modern JVM class libraries.
Add plumbing to tests.xml to allow test-specific settings of the
VM's bootclasspath and then use this mechanism in the dacapo build.xml
to ensure that the xalan code taken from the xalan.jar extracted from
dacapo.jar is loaded in preference to whatever xalan version may be
built-in to the classlibarary.