Remove BeastFX as a test dependency
The test suite should not require BeastFX to run. To that end, we
trigger the XML parser and then invoke beast.base.inference.MCMC
ourselves instead of relying on beastfx.app.beast.BeastMain
for that.
The latter is however still the entry point for the jar file and thus the Ant "run" target. So BeastFX remains a run-time dependency when using Lumière as a stand-alone application. That too could be changed, just like for the test suite. It would remove the dependency on JavaFX, which makes it easier to deploy Lumière, as it could then run in any Java 17+ environment.