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/beacon_chain/tests/block_verification.rs

@@ -297,19 +297,20 @@ async fn chain_segment_full_segment() {
 
 #[tokio::test]
 async fn chain_segment_varying_chunk_size() {
-    for chunk_size in &[1, 2, 3, 5, 31, 32, 33, 42] {
+    let (chain_segment, chain_segment_blobs) = get_chain_segment().await;
+    let blocks: Vec<RpcBlock<E>> = chain_segment_blocks(&chain_segment, &chain_segment_blobs)
+        .into_iter()
+        .collect();
+
+    for chunk_size in &[1, 2, 31, 32, 33] {
         let harness = get_harness(VALIDATOR_COUNT, NodeCustodyType::Fullnode);
-        let (chain_segment, chain_segment_blobs) = get_chain_segment().await;
-        let blocks: Vec<RpcBlock<E>> = chain_segment_blocks(&chain_segment, &chain_segment_blobs)
-            .into_iter()
-            .collect();
 
         harness
             .chain
             .slot_clock
             .set_slot(blocks.last().unwrap().slot().as_u64());
 
-        for chunk in blocks.chunks(*chunk_size) {
+        for chunk in blocks.clone().chunks(*chunk_size) {
             harness
                 .chain
                 .process_chain_segment(chunk.to_vec(), NotifyExecutionLayer::Yes)