test: ics20 send_transfer using mock chains

[rnbguy]

Part of: #5

Description

Implements an ics20 test using mocks of a cosmos-based chain and a sovereign chain.

With this PR:

• ✅ Mock Cosmos (send_transfer)  > ✅ Mock Rollup (recv_packet)
• ✅ Mock Rollup (send_transfer) > ⚠️ Mock Cosmos (recv_packet): To make the recv_packet possible, we must refactor the TestApp abstraction first.

---

PR author checklist

• Linked to GitHub issue.
• Added tests.
• Updated code comments and documentation (e.g., docs/).
• Tagged one reviewer who will be the one responsible for shepherding this PR.

[Farhad-Shabani]

Thanks @rnbguy for this awesome work.
I've been tinkering around to make the current abstracted mock Sovereign do its job – growing some blocks/headers and update the root states of the rollup - So can fetch the proofs of commitment, craft the MsgRecvPacket, shoot it over to the mock Cosmos chain and nail the token transfer tests in both directions.
But to make all this happen, I should give the mock Sovereign makeover. Wrapping up what we've done so far in this PR, and then diving into that refactoring.