Error: Unhandled exception: Asynchronous task [sendMessageSlow() at pubsubpeer.nim:301] was cancelled! [FutureDefect]
**Describe the bug**
Nimbus validator node crashed during normal operation. Note strange inclusion of "/nim-testutils/testutils/moduletests.nim(21) moduletests" in stack, as well as "[FutureDefect]" (a self-fulfilling prophecy, that one).

**To Reproduce**
Steps to reproduce the behavior:
1. Platform details (OS, architecture): Ubuntu 22.04.4 amd64
2. Branch/commit used: nimbus-eth2_Linux_amd64_24.4.0_f20a21c0; geth-linux-amd64-1.14.0-87246f3c
3. Commands being executed: `./run-mainnet-beacon-node.sh --rest --subscribe-all-subnets --nat:extip:x.x.x.x --jwt-secret=/home/cleduc/geth-mainnet-secret.jwt --el=http://127.0.0.1:8551 --payload-builder=true --payload-builder-url=http://127.0.0.1:18550 --suggested-fee-recipient=0x... --metrics --metrics-port=8008 --doppelganger-detection=off`
4. Relevant log lines:
```
INF 2024-05-08 12:58:03.819+00:00 Attestation sent                         topics="message_router" attestation="(aggregation_bits:0b000000000000000000000000...000, data: (slot: 9029088, index: 3, beacon_block_root: \"6028bdbe\", source: \"282158:d011bbcd\", target: \"282159:6028bdbe\"), signature: \"865edc5d\")" delay=819ms304us800ns subnet_id=3
peers: 162 > finalized: 5c050910:282157 > head: 6028bdbe:282159:0 > time: 282159:0 (9029088) > sync: synced
ETH: 32.000112 /home/user/nimbus-eth2/vendor/nim-testutils/testutils/moduletests.nim(21) moduletests
/home/user/nimbus-eth2/beacon_chain/nimbus.beacon.node.nim(2392) main
/home/user/nimbus-eth2/beacon_chain/nimbus.beacon.node.nim(2315) handleStartUpCmd
/home/user/nimbus-eth2/beacon_chain/nimbus.beacon.node.nim(2392) doRunBeaconNode
/home/user/nimbus-eth2/beacon_chain/nimbus.beacon.node.nim(2392) start
/home/user/nimbus-eth2/beacon_chain/nimbus.beacon.node.nim(2392) run
/home/user/nimbus-eth2/vendor/nim-chronos/chronos/internal/asyncengine.nim(150) poll
Error: Unhandled exception: Asynchronous task [sendMessageSlow() at pubsubpeer.nim:301] was cancelled! [FutureDefect]
```

**Screenshots**
If applicable, add screenshots to help explain your problem.
![Screenshot from 2024-05-08 15-40-32](https://github.com/status-im/nimbus-eth2/assets/23334/78f6c054-af2f-4783-a420-1cb7927c6aeb)


**Additional context**
Node was running uninterrupted through two consecutive sync committees.
