add deploy mock erc20 to utils

[ItayRosenberg]

Note

Adds an ERC20 test utility to deploy a mock token and refactors testing utils to use it.

• Utils:
  • Add packages/utils/src/erc20/test_utils.cairo with deploy_mock_erc20_contract.
  • Export erc20::test_utils behind #[cfg(test)].
• Testing:
  • TokenDeployImpl in packages/testing/src/test_utils.cairo now uses deploy_mock_erc20_contract instead of manual declare/deploy.
  • Streamline imports accordingly.

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