Just found the easy solution, it is to actually do set the quarkus.datasource.username:

quarkus.flyway.migrate-at-start=true
quarkus.flyway.schemas=oracletest
quarkus.datasource.username=oracletest

That may be obvious when comparing it with a production environment where schema name and user name are the same. In my case of an integration test environment based on devservices it took me some time to find out.