Update to spec v1.1.8 (#2893)

## Proposed Changes

Change the canonical fork name for the merge to Bellatrix. Keep other merge naming the same to avoid churn.

I've also fixed and enabled the `fork` and `transition` tests for Bellatrix, and the v1.1.7 fork choice tests.

Additionally, the `BellatrixPreset` has been added with tests. It gets served via the `/config/spec` API endpoint along with the other presets.

testing/ef_tests/src/cases/operations.rs

@@ -239,7 +239,6 @@ impl<E: EthSpec> Operation<E> for ExecutionPayload<E> {
         spec: &ChainSpec,
         extra: &Operations<E, Self>,
     ) -> Result<(), BlockProcessingError> {
-        // FIXME(merge): we may want to plumb the validity bool into state processing
         let valid = extra
             .execution_metadata
             .as_ref()