You can use Quarkus profiles to change which Cassandra database your tests connect to. For example,
%prod.quarkus.cassandra.contact-points=my-cassandra:9042
%dev.quarkus.cassandra.contact-points=127.0.0.1:9042
That might get you part of the way. As Erick Ramirez says, completely disabling the Cassandra parts of the application will have some downsides in terms of test coverage (and test realism). For many Quarkus extensions, there are dev services which will automatically stand up an instance of the dependency when tests are run, and in dev mode. It doesn't look like that's been implemented for the Cassandra extension yet. As an alternative, you might be able to use the testcontainers Cassandra support. It's more work than having Quarkus dev services do it 'for free', but not impossible.