Contains some funny mocking logic, because estimate gas price is called
multiple times (for various nblocks) and I wanted to make it as flexible
as possible.
Also moved the mock definition to a separate file (mocks_test.go)
because it's gonna be used in other test files, and it didn't make sense
for it to stay inside chain_test.go.
CLI actions lack unit tests. I decided to use the approach similar to
what I found in `send_test.go` using gomock, but I don't rely on custom
"service" implementations but mock the whole FullNode API.
This first commit validates the test setup by testing the simplest method
of the chain category, e.g. `chain head`.
This requires a minor refactor of the CLI action code:
- The constructor (`GetFullNodeAPI`) checks if there's an injected mock
API in the app Metadata and uses that in unit tests.
- Actions shouldn't use raw `fmt.*` but instead write to the `app.Writer`
so the CLI output is testable
