Add Gloas boilerplate (#7728)

Adds the required boilerplate code for the Gloas (Glamsterdam) hard fork. This allows PRs testing Gloas-candidate features to test fork transition.

This also includes de-duplication of post-Bellatrix readiness notifiers from #6797 (credit to @dapplion)

testing/ef_tests/src/handler.rs

@@ -22,7 +22,7 @@ pub trait Handler {
     // Add forks here to exclude them from EF spec testing. Helpful for adding future or
     // unspecified forks.
     fn disabled_forks(&self) -> Vec<ForkName> {
-        vec![]
+        vec![ForkName::Gloas]
     }
 
     fn is_enabled_for_fork(&self, fork_name: ForkName) -> bool {