Fix flaky zombienet test `zombienet-polkadot-functional-0017-sync-backing`

[pepoviola]

Some examples from Tuesday.

https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7786202 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7786140 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7785874 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7785813 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7785747 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7785682 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7785615 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7785554 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7785489 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7785292 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7785227 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7785162 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7785033 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7784901 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7784836 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7784710 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7784378 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7784310 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7784245 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7784180 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7784099 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7784017 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7783952 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7783887 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7783822 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7783358 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7782804 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7782675 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7782609 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7782386 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7782312 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7782182 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7782117 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7782052 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7781984 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7781919 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7781771 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7781227 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7780939 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7780877 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7780545 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7780336 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7780078 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7780011 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7779885 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7779816 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7779809 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7779702 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7779636 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7779575 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7779512 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7779447 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7779320 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7779191 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7779124 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7779058 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7778993 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7778926 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7778859 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7778391 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7778326 https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7778261

@paritytech/parachains-core

[ordian]

cc @sandreim @alindima