TestUnmanagedAMLauncher can corrupt target/test-classes/yarn-site.xml during write phase, breaks later test runs

the setup phase of TestUnmanagedAMLauncher overwrites yarn-site.xml. As Configuration.writeXml() does a reread of all resources, this will break if the (open-for-writing) resource is already visible as an empty file.

This leaves a corrupted target/test-classes/yarn-site.xml, which breaks later test runs -because it is not overwritten by later incremental builds, due to timestamps.