Move AppendRandomString to pkg

[adrcunha]

Expected Behavior

This useful helper function is shared across all repos through pkg.

Actual Behavior

This function is duplicated in several repos, like in https://github.com/knative/build-pipeline/blob/master/test/randstring.go (with test) and https://github.com/knative/serving/blob/02d3c9b657fdefe76690a1d5751ae0b2c4e414ae/test/crd.go#L255