Refactor "test-sol" directory structure into unit, e2e, and integration tests

[arthurgousset]

Follow-on task from this PR:

• https://github.com/celo-org/celo-monorepo/pull/11020

I decided to keep that PR small, but in a subsequent PR, I'd like to refactor the test-sol/ directory structure like:

test-sol/unit/
test-sol/e2e/
test-sol/integration/

That way we could run tests on CI with a command structure like:

# unit tests
forge test --match-path "*test-sol/unit/*"

# e2e tests
forge test --match-path "*test-sol/e2e/*"

# integration tests
forge test --match-path "*test-sol/integration/*"

Nice to haves

1. Consider refactoring this step on CI:

   1. have better file name
   2. run a single anvil devchain to use for integration and e2e tests to save time

   https://github.com/celo-org/celo-monorepo/blob/bfbad45621bd5d5ac5cb35de1b005ef126c6b996/.github/workflows/protocol_tests.yml#L102-L104

2. Move anvil scripts to scripts/anvil/ instead of migrations_sol/ (we already have scripts/truffle/ and scripts/bash/)

3. Organise CI workflow into unit, integration, and e2e tests

   https://github.com/celo-org/celo-monorepo/blob/bfbad45621bd5d5ac5cb35de1b005ef126c6b996/.github/workflows/protocol_tests.yml#L64-L65

   - name: Run tests common
   + name: Run unit tests common