Reduce number of blobs used in tests to speed up CI (#8194)

`beacon-chain-tests` is now regularly taking 1h+ on CI since Fulu fork was added.

This PR attemtpts to reduce the test time by bringing down the number of blobs generated in tests - instead of generating 0..max_blobs, the generator now generates 0..1 blobs by default, and this can be modified by setting `harness.execution_block_generator.set_min_blob_count(n)`.

Note: The blobs are pre-generated and doesn't require too much CPU to generate however processing a larger number of them on the beacon chain does take a lot of time.

This PR also include a few other small improvements
- Our slowest test (`chain_segment_varying_chunk_size`) runs 3x faster in Fulu just by reusing chain segments
- Avoid re-running fork specific tests on all forks
- Fix a bunch of tests that depends on the harness's existing random blob generation, which is fragile


beacon chain test time on test machine is **~2x** faster:

### `unstable`

```
Summary [ 751.586s] 291 tests run: 291 passed (13 slow), 0 skipped
```

### this branch

```
Summary [ 373.792s] 291 tests run: 291 passed (2 slow), 0 skipped
```

The next set of tests to optimise is the ones that use [`get_chain_segment`](77a9af96de/beacon_node/beacon_chain/tests/block_verification.rs (L45)), as it by default build 320 blocks with supernode - an easy optimisation would be to build these blocks with cgc = 8 for tests that only require fullnodes.


  


Co-Authored-By: Jimmy Chen <jchen.tc@gmail.com>

Co-Authored-By: Jimmy Chen <jimmy@sigmaprime.io>

beacon_node/http_api/tests/broadcast_validation_tests.rs

@@ -822,6 +822,14 @@ pub async fn blinded_gossip_invalid() {
 
     tester.harness.advance_slot();
 
+    // Ensure there's at least one blob in the block, so we don't run into failures when the
+    // block generator logic changes, as different errors could be returned:
+    // * Invalidity of blocks: `NotFinalizedDescendant`
+    // * Invalidity of blobs: `ParentUnknown`
+    tester
+        .harness
+        .execution_block_generator()
+        .set_min_blob_count(1);
     let (blinded_block, _) = tester
         .harness
         .make_blinded_block_with_modifier(chain_state_before, slot, |b| {
@@ -837,21 +845,20 @@ pub async fn blinded_gossip_invalid() {
     assert!(response.is_err());
 
     let error_response: eth2::Error = response.err().unwrap();
+    assert_eq!(error_response.status(), Some(StatusCode::BAD_REQUEST));
+
     let pre_finalized_block_root = Hash256::zero();
-    /* mandated by Beacon API spec */
-    if tester.harness.spec.is_fulu_scheduled() {
-        // XXX: this should be a 400 but is a 500 due to the mock-builder being janky
-        assert_eq!(
-            error_response.status(),
-            Some(StatusCode::INTERNAL_SERVER_ERROR)
-        );
+    let expected_error_msg = if tester.harness.spec.is_fulu_scheduled() {
+        format!(
+            "BAD_REQUEST: NotFinalizedDescendant {{ block_parent_root: {pre_finalized_block_root:?} }}"
+        )
     } else {
-        assert_eq!(error_response.status(), Some(StatusCode::BAD_REQUEST));
-        assert_server_message_error(
-            error_response,
-            format!("BAD_REQUEST: ParentUnknown {{ parent_root: {pre_finalized_block_root:?} }}"),
-        );
-    }
+        // Since Deneb, the invalidity of the blobs will be detected prior to the invalidity of the
+        // block.
+        format!("BAD_REQUEST: ParentUnknown {{ parent_root: {pre_finalized_block_root:?} }}")
+    };
+
+    assert_server_message_error(error_response, expected_error_msg);
 }
 
 /// Process a blinded block that is invalid, but valid on gossip.
@@ -1647,6 +1654,10 @@ pub async fn block_seen_on_gossip_with_some_blobs_or_columns() {
         )
         .await;
     tester.harness.advance_slot();
+    tester
+        .harness
+        .execution_block_generator()
+        .set_min_blob_count(2);
 
     let slot_a = Slot::new(num_initial);
     let slot_b = slot_a + 1;