There is no need for a test case to understand this issue.
But for completeness I downloaded the latest stable version: hibernate-core-4.3.5.Final

In package "org.hibernate.dialect" none of the following classes have the "concat_ws" MySQL function:
MySQL5Dialect
MySQL5InnoDBDialect
MySQLDialect
MySQLInnoDBDialect
MySQLMyISAMDialect

Only org.hibernate.dialect.MySQLDialect has its sister function:

registerFunction("concat", new StandardSQLFunction("concat", StandardBasicTypes.STRING));