Bridge zombienet tests: remove unneeded accounts

Related to https://github.com/paritytech/polkadot-sdk/issues/3475

Related to https://github.com/paritytech/polkadot-sdk/issues/3400

Moving all bridges testing "framework" files under one folder in order
to be able to download the entire folder when we want to add tests in
other repos

No significant functional changes

Related to https://github.com/paritytech/polkadot-sdk/issues/3242