Add tests for gRPC methods

[arya2]

Motivation

We want to have tests in CI to check that gRPC methods are working as expected.

Note: There's already test coverage for most of the logic in methods that have been implemented, and the gRPC methods have been manually tested, this issue is for filling the remaining gaps.

Possible Design

• Snapshot tests like those in zebra-rpc
• Acceptance tests (See the changes in zebrad/tests/acceptance.rs here for calling these methods from acceptance tests)

[oxarbitrage]

Snapshots tests for the methods we had written were done at #8277
Other snapshots tests will be added in the PR of the method itself (For example #8268)

Unit tests foer the grpc methods will be done in a separated PR that will close this ticket.