Add missing directories after maven import

Add missing directory, src/test/java after maven import.

create "src/test/java" if missing

Reproduce the Maven import in the gwt-eclipse-plugin project and observe whether src/test/java is absent afterward. Trace the import setup or project-generation entry point responsible for creating directories. Done means the src/test/java directory exists after import when it was previously missing.

索引モデルが issue の本文から書いたものです。