Ok, after some unfruitful debugging sessions, as a last resort, I simply wiped all third party code (dependencies) from the workspace:
cd $GOPATH rm -rf pkg/ bin/ src/github.com src/golang.org # the idea is to remove all except your own source
go get to get all used dependencies again:
go get github.com/yadayada/yada go get # etc
And the problem is gone! Application is starting normally and tests are passing. No startup panics anymore. It looks like this problem happens when you
mv your work space folder but I'm not 100% sure yet. Hope it helps someone else.
From now on, re install dependencies will be my first step when weird panic conditions like that suddenly appear.