Add tests for multi-denom transfer

[gjermundgaraba]

In #114, we added support for ICS20v2, which has multi-denom support. It means it can transfer and receive multiple denoms at a time, which needs proper testing.

Add at least the following tests:

• Unit case in test_newMsgSendPacketV2
• Unit tests (happy path and at least one failure case) for both send and receive in ICS20TransferTest.t.sol
• Unit tests for both send and receive in IntegrationTests.t.sol
• E2E test with both send and receive (e.g. Eth -> Cosmos -> Eth)

[srdtrk]

We won't be including ICS20-v2 in the initial release