test: consolidate mocking frameworks

PR https://github.com/babylonlabs-io/optimism/pull/8 and https://github.com/babylonlabs-io/optimism/pull/7 added mocks for the finality gadget client using the [`gomock`](https://github.com/uber-go/mock) framework

However, existing tests use [`mockery`](https://github.com/vektra/mockery).

We should consolidate mocking frameworks to be consistent with the existing OP codebase 