Prepare LegacyInjector.preinit() for the upcoming imagej-legacy

The new imagej-legacy version will change the startup, requiring the
PluginClassLoader to be disabled.

The pom-imagej version we use as our parent currently was already
prepared for the new imagej-legacy version, so let's just sneak this
change into the 'test-utils' branch that already bumps the parent.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
TO-UNDO