Fix flaky zombienet test `zombienet-polkadot-functional-0003-beefy-and-mmr`

[pepoviola]

Examples (from yesterday 22 of 79 fails):

https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7528805 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7528419 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7527903 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7527008 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7526541 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7525278 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7525003 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7524716 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7524343 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7523961 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7522737 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7522003 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7521826 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7521465 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7521186 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7520907 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7520731 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7520537 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7518897 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7518528 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7518263 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7517848

cc: @paritytech/bridges-core

[serban300]

@pepoviola Thank you for the report ! I can assign it to myself and will take a look on it these days