Cover Linkdrop Escrow Contracts with tests

[mikhaildobs]

Contracts to test:

• - ERC20 - https://github.com/LinkdropProtocol/volca-linkdrop-core/blob/master/contracts/e2pAirEscrow.sol
• - ERC721 - https://github.com/LinkdropProtocol/volca-linkdrop-core/blob/master/contracts/LinkdropNFT.sol

Tests should cover similar cases as the one-to-one linkdrops tests - https://github.com/LinkdropProtocol/Linkdrop-Webapp/blob/master/test/e2pEscrow.js

[amiromayer]

✅ Covered ERC20 contract

[amiromayer]

✅ Covered ERC721 contract