yang-maven-plugin-it hardcodes the target directory

In the file yang/yang-maven-plugin-it/src/test/java/org/opendaylight/yangtools/yang2sources/plugin/it/YangToSourcesPluginTestIT.java the build directory is assumed to be "target" instead that could have been relocated by setting the project.build.directory.

The bug is opened to remove this hardcoding and move to looking up the project.build.directory