feat(backend ci): run unit test on push + check coverage

[Tangui-Bitfly]

• run unit test and check coverage threshold on each push that modify backend
• skip pkg/consapi and pkg/api because they are not working right now
• skip internal/th it's a test helper package
• skip internal/contracts and backend/pkg/commons/types because it's mostly generated code

Feel free to suggest other packages to skip

[mccreedy-bitfly]

lgtm.