[Go][design] Define what's not unit test

Reference:

https://twitter.com/davecheney/status/1178289934542655489

A test is not a unit test if:

• It talks to the database

• It communicates across the network

• It touches the file system 

• It can't run at the same time as any of your other unit tests 

• You have to do special things to your environment to run it.