Pre-generate test blobs bundle to improve test time. (#4829)

## Issue Addressed

Addresses #4778, and potentially fixes the flaky deneb builder test `builder_works_post_deneb`.

The [deneb builder test](c5c84f1213/beacon_node/http_api/tests/tests.rs (L5371)) has been quite flaky on our CI (`release-tests`) since it was introduced. I'm guessing that it might be timing out on the builder `get_header` call (1 second), and therefore the local payload is used, while the test expects builder payload to be used. 

On my machine the [`get_header` ](c5c84f1213/beacon_node/execution_layer/src/test_utils/mock_builder.rs (L367)) call takes about 550ms, which could easily go over 1s on slower environments (our windows CI runner is much slower than the ubuntu one).

I did a profile on the test and it showed that `blob_to_kzg_commiment` and `compute_kzg_proof` was taking a large chunk of time, so perhaps pre-generating the blobs could help stablise this test.

## Proposed Changes

Pre-generate blobs bundle for Mainnet and Minimal presets.

Before the change `get_header` took about **550ms**, and it's now reduced to **50-55ms** after the change. If timeout was indeed the cause of the flaky test, this fix should stablise it. This also brings the flaky `builder_works_post_deneb` test time from 50s to 10s. (8s if we only use a single blob)

beacon_node/beacon_chain/src/test_utils.rs

@@ -2526,7 +2526,6 @@ pub enum NumBlobs {
 pub fn generate_rand_block_and_blobs<E: EthSpec>(
     fork_name: ForkName,
     num_blobs: NumBlobs,
-    kzg: &Kzg<E::Kzg>,
     rng: &mut impl Rng,
 ) -> (SignedBeaconBlock<E, FullPayload<E>>, Vec<BlobSidecar<E>>) {
     let inner = map_fork_name!(fork_name, BeaconBlock, <_>::random_for_test(rng));
@@ -2540,8 +2539,7 @@ pub fn generate_rand_block_and_blobs<E: EthSpec>(
             NumBlobs::None => 0,
         };
         let (bundle, transactions) =
-            execution_layer::test_utils::generate_random_blobs::<E, _>(num_blobs, kzg, rng)
-                .unwrap();
+            execution_layer::test_utils::generate_blobs::<E>(num_blobs).unwrap();
 
         payload.execution_payload.transactions = <_>::default();
         for tx in Vec::from(transactions) {