JSON-RPC mock unit tests

[fedekunze]

Create mock tests for the JSON-RPC (/rpc dir) using the testify mock package.

See also Tendermint RPC mock suite for reference on how to create mock RPC tests:

• https://github.com/tendermint/tendermint/blob/master/rpc/client/mocks/client.go
• https://github.com/tendermint/tendermint/blob/master/rpc/client/mock/abci_test.go

Evmos mock tests ref:

• imp: boilerplate for IBC modules evmos#412

JSON-RPC mock tests:

• https://github.com/LordForkuaad/go-evm-client