fix: gate the test crate behind the "unstable" feature
fix: gate the test crate behind the "unstable" feature

Add a configuration flag to the `test` crate import so that the crate is only
imported when the "unstable" feature is enabled (i.e. when building on nightly).

uiri/SEGIMAP,uiri/SEGIMAP,indiv0/SEGIMAP,indiv0/SEGIMAP,uiri/SEGIMAP,indiv0/SEGIMAP