Bridge: fix zombienet tests (#4367)

Due to recent bump of Rococo/Westend versions + the fact that
https://github.com/paritytech/parity-bridges-common/pull/2894 has
finally reached this repo, tests now fail, because we've started
checking all client versions (even source) unless we specify
`--source-version-mode Auto` in CLI arguments. This looks like an
overkill, but all those version checks will be fixed by
https://github.com/paritytech/polkadot-sdk/pull/4256, so now it makes
sense just to add this CLI option. We also need to propagate it to
running relays eventually.