testing(json rpc): Add backend test suite with mock grpc query client

[danburck]

Description

This PR adds a backend testing suite in order to unit test the JSON-RPC backend. It uses a mock for the grpc client to create the following setup:

JSON-PRC-CLIENT -> BACKEND -> Mock GRPC CLIENT -> APP

where

• a JSON-PRC-CLIENT doesn't need to be instantiated because we directly call the backend methods
• APP doesn't need to be instantiated because the Mock GRPC CLIENT response with fake data

Notes

• I included a first unit test. After this setup is merged we need to add more coverage
• An alternative to a mocked grpc, would be to run a grpc server, that would mimic the actual grpc response.

[codecov]

Codecov Report

Merging #1199 (3b6e368) into main (cd41c48) will decrease coverage by 7.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1199      +/-   ##
==========================================
- Coverage   62.32%   55.29%   -7.03%     
==========================================
  Files          91       94       +3     
  Lines        7413     8391     +978     
==========================================
+ Hits         4620     4640      +20     
- Misses       2563     3517     +954     
- Partials      230      234       +4     

Impacted Files	Coverage Δ
rpc/backend/evm_backend.go	0.99% <0.00%> (ø)
rpc/backend/backend.go	86.66% <0.00%> (ø)
rpc/backend/utils.go	0.00% <0.00%> (ø)