sbt tests are intermittenly hanging

Starting at commit c81b213b02ac414a39290787cf2eea14d83fdc26, running {sbt daffodil-io/test} multiple times will eventually cause it to hang.

It's not clear to me the cause, but I've confirmed that the hang happens somewhere in the SimpleNamedServiceLoader loadClass function. Seems likely that it's getting stuck in the while loop and can't break out for some reason.

I don't think the above commit caused the issue, but it did a new call to SimpleNamedServiceLoader, and it loads a lot more classes than the usual use of this class. We're probably using service loaders incorrectly in this function somehow.