TODO: Improve State API tests

[tclemos]

Rationale

State API has some tests that are asserting results in a "not so precise" validation, it needs some improvements as we can see here:

• https://github.com/hermeznetwork/hermez-node/blob/3fc5b4c531394e9d6a9975e2fa798bbd5029e6a4/api/state_test.go#L206
• https://github.com/hermeznetwork/hermez-node/blob/3fc5b4c531394e9d6a9975e2fa798bbd5029e6a4/api/state_test.go#L211
• https://github.com/hermeznetwork/hermez-node/blob/3fc5b4c531394e9d6a9975e2fa798bbd5029e6a4/api/state_test.go#L219

Implementation

Change test asserts to be more precise in the value assertion.