The test suite no longer requires 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 run in any Java 17+
environment.