Introduce faillint in the CI workflow

[pracucci]

What this PR does:
A common mistake (I did it too) is to import github.com/bmizerany/assert instead of github.com/stretchr/testify/assert. In this PR I've introduced faillint as further lint step to ensure no blacklisted package is imported. Of you try to use github.com/bmizerany/assert the linter will fail, suggesting which package should be used instead:

/go/src/github.com/cortexproject/cortex/pkg/storage/tsdb/config_test.go:8:2: package "github.com/bmizerany/assert" shouldn't be imported, suggested: "github.com/stretchr/testify/assert"

faillint will also be useful in the future to enforce some design patterns (ie. once completed the Prometheus metrics refactoring, we'll be able to forbid the usage of prometheus.NewGauge() in favor of promauth.With(reg).NewGauge() and so on).

Which issue(s) this PR fixes:
N/A

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[pstibrany]

Thanks. This is useful, and we can extend it with more packages (eg. old context package), in the future.