Manage the plexus-utils dependency in the testbom

Various testsuite modules in WildFly Core use org.codehaus.plexus:plexus-utils, but there is no use in production code. But core is managing the dependency via its root pom. Core shouldn't be exposing test-only dependencies in the root pom; those should be managed via its testbom model.

As part of this consider bumping the version which is far behind the current.