Shorten execution time of E2E tests

[Stefan-Ethernal]

Description

Changelog:

• Provided a smaller epoch size, compared to the default one in basic consensus tests.
• Skip TestE2E_Consensus_Sync_WithNonValidators, until we implement proper support for non-validator nodes in E2E framework.
• Move test helper functions to helpers_test.go.
• Implement waitForRootchainEpoch helper function, which checks whether target epoch is reached.
• Merged TestE2E_Bridge_DepositAndWithdrawERC20 and TestE2E_Bridge_MultipleCommitmentsPerEpoch into TestE2E_Bridge_Transfers test (resulted in ~2 mins faster execution).
• Merged TestE2E_Consensus_Basic_WithNonValidators and TestE2E_Consensus_Sync into TestE2E_Consensus_Basic_WithNonValidators (resulted in ~7 secs faster execution).

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually