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>

testing/node_test_rig/src/lib.rs

@@ -248,14 +248,8 @@ impl<E: EthSpec> LocalExecutionNode<E> {
         if let Err(e) = std::fs::write(jwt_file_path, config.jwt_key.hex_string()) {
             panic!("Failed to write jwt file {}", e);
         }
-        let spec = context.eth2_config.spec.clone();
         Self {
-            server: MockServer::new_with_config(
-                &context.executor.handle().unwrap(),
-                config,
-                spec,
-                None,
-            ),
+            server: MockServer::new_with_config(&context.executor.handle().unwrap(), config, None),
             datadir,
         }
     }