[SC-440] Refactor E2E

[hexonaut]

Refactoring the E2E cross-domain testing to be generalized. In particular:

• Split out Bridging from Domains - Domain = blockchain (IE Ethereum, Optimism, Solana, etc), Bridge = Optimism Native Bridge, Circle CCTP, etc
• Don't assume anchoring on Ethereum
• Drop the L1/L2 language it should just be source and destination domains

[lucas-manuel]

Definitely want to see @barrutko's review on this but this looks good to me as is, I reviewed:

• Testing file diffs (minor changes)
• DomainHelpers
• Bridge

Then I diffed src/test/bridges/* against their *Domain.sol counterparts that were removed as part of this PR. Changes all made sense to me and tests all pass so LGTM after this small change.

[github-actions]

Coverage after merging SC-440-refactor-e2e into master will be

70.45%

Coverage Report

File	Stmts	Branches	Funcs	Lines	Uncovered Lines
src
ArbitrumReceiver.sol	80%	100%	75%	75%	15
CCTPReceiver.sol	96%	90%	100%	100%	43
GnosisReceiver.sol	77.78%	100%	75%	62.50%	24–26
OptimismReceiver.sol	84.62%	100%	75%	80%	21
XChainForwarders.sol	50%	100%	50%	50%	122, 139, 169, 186, 198, 241, 70, 83