Add more tests

[mostafa]

Target is at least 80% coverage. The coverage report (on coveralls.io) shows which parts needed test the most. It's mostly about functionality and unhappy path testing (in error cases).

• [x] gRPC server and HTTP gateway
• [ ] Plugin Registry (load plugin, register hooks, etc.): needs more work, because there needs to be a plugin to load and register its hooks.
• [ ] Network proxy: needs more work in other milestones, especially after #398.

Depends

• https://github.com/gatewayd-io/gatewayd/issues/439