Run Ant tests based on items included in the build

[JosephCottam]

For example, don't run the extension tests when only the core jar was built.

[JosephCottam]

Requires introspection on the contents of the loaded jar. Can be achieved by either attempting to load specific class files by name before executing tests. This check might be achieved in ant through the "available" task. (Though the jar file might need to be unzipped...and related cleanup steps.)