Is it possible that this information (during tests ) is from classpath like an spi?

No this is not what happens (although there is the idea to do something like this, see OGM-544). The provider needs to explicitly configured via the "hibernate.ogm.datastore.provider" property in persistence.xml or - as in this case - hibernate.properties. Here is the one used for MongoDB tests: https://github.com/hibernate/hibernate-ogm/blob/master/mongodb/src/test/resources/hibernate.properties

So a different one would have to be used for Fongo. Maybe via some Maven profile?