Optimise slow block verification tests

[jimmygchen]

Issue Addressed

Reduce CI time for block_verification tests that exceed 120s on Fulu/Gloas forks.

• Cache the chain segment across tests using LazyLock<OnceCell> (was rebuilt 10+ times independently)
• Reduce chain length from 320 to 192 slots (10 epochs to 6 epochs)
• Reduce BLOCK_INDICES from 7 to 3
• Reduce chain_segment_varying_chunk_size from 5 to 3

AI assisted, self reviewed.

Before vs After comparison (1 sample only):

• Before: 19 tests exceeded the 120s slow threshold.
• After: zero exceed it. Overall: 1,890s down to 797s (-58%).

| Test                                                 |  Before |   After |  Change |
|------------------------------------------------------|---------|---------|---------|
| chain_segment_varying_chunk_size                     |   239s  |     98s |    -59% |
| invalid_signature_attester_slashing                  |   175s  |     64s |    -64% |
| invalid_signature_exit                               |   173s  |     62s |    -64% |
| invalid_signature_deposit                            |   170s  |     60s |    -65% |
| invalid_signature_attestation                        |   165s  |     62s |    -63% |
| invalid_signature_proposer_slashing                  |   161s  |     56s |    -66% |
| block_gossip_verification                            |   154s  |     91s |    -41% |
| invalid_signature_block_proposal                     |   151s  |     58s |    -61% |
| invalid_signature_randao_reveal                      |   149s  |     54s |    -64% |
| invalid_signature_gossip_block                       |   135s  |     46s |    -66% |
|------------------------------------------------------|---------|---------|---------|
| TOTAL                                                |  1890s  |    797s |    -58% |

[pawanjay176]

LGTM

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-06-05 19:26 UTC · Rule: default
• ✅ Checks started — 2026-06-05 19:28 UTC · on draft merge queue: embarking unstable (e78e1d3) and [#9418 + #9131 + #9274] together #9424
• ❌ Checks failed
• ✂️ Bisecting to identify the failing PR (round 1/1)
• 🚫 Left the queue — 2026-06-05 20:16 UTC · at 361cd18d756bdda384e420e01115232f02e28396

This pull request spent 50 minutes 4 seconds in the queue, including 19 minutes 37 seconds running CI.

Reason

The merge conditions cannot be satisfied due to failing checks

Hint

You may have to fix your CI before adding the pull request to the queue again.
If you update this pull request, to fix the CI, it will automatically be requeued once the queue conditions match again.
If you think this was a flaky issue instead, you can requeue the pull request, without updating it, by posting a @mergifyio queue comment.

[pawanjay176]

@mergify requeue

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-06-05 20:21 UTC · Rule: default
• ✅ Checks passed · on draft merge queue: embarking unstable (abe7ca2) and #9274 together #9427
• ✅ Merged — 2026-06-05 20:51 UTC · at 361cd18d756bdda384e420e01115232f02e28396 · squash

This pull request spent 30 minutes 33 seconds in the queue, including 28 minutes 48 seconds running CI.

Required conditions to merge

• #approved-reviews-by >= 1 [🛡 GitHub branch protection]
  • Optimise slow block verification tests #9274
• check-success=local-testnet-success
• check-success=test-suite-success