Increase test coverage to 80%

[Nezteb]

Describe the bug As part of adding ergo to awesome-go, the repo needs to have at least 80% test coverage, which it currently does not.

Current test coverage: coverage.zip

❯ cat coverage.html | grep '<option value="file' | sed -E 's/.*>(.*) \((.*)%\)<.*/\2 \1/' | sort -rn

89.2 github.com/ergo-services/ergo/etf/encode.go
65.8 github.com/ergo-services/ergo/etf/etf.go
56.2 github.com/ergo-services/ergo/lib/mpsc.go
51.0 github.com/ergo-services/ergo/etf/decode.go
43.8 github.com/ergo-services/ergo/etf/cache.go
12.7 github.com/ergo-services/ergo/proto/dist/proto.go
8.3 github.com/ergo-services/ergo/lib/tools.go
0.0 github.com/ergo-services/ergo/proto/dist/registrar.go
0.0 github.com/ergo-services/ergo/proto/dist/handshake.go
0.0 github.com/ergo-services/ergo/proto/dist/flusher.go
0.0 github.com/ergo-services/ergo/proto/dist/epmd.go
0.0 github.com/ergo-services/ergo/lib/cert.go