test: initialize submodules before local suite

[newmattock]

Summary:

• Initializes required Git submodules before running the local multichain test suite.
• Documents the submodule setup step in the README test instructions.
• Uses an EXIT trap so the Docker Compose stack is torn down if tests fail.

Related bounty or issue:

• BGL (Bitgesell) Bounty/Improvement Program ($50000 budget) bitgesell#81

What changed:

• test.sh now runs git submodule update --init --recursive when executed inside a Git checkout.
• test.sh now exits on failures and keeps docker-compose down registered as cleanup.
• README test setup now calls out the required submodule initialization for existing checkouts.

Validation:

• bash -n test.sh
• git diff --check

Notes:

• I reproduced that a plain clone leaves chain/filecoin/filecoin-ffi and chain/solana/solana-ffi uninitialized, which makes the Go module replacement for github.com/renproject/solana-ffi resolve to a missing local go.mod before tests can run.
• I did not run the full Docker-backed suite here.