Consitently switch to stretchr/testify

We were using stretchr/testify and test-go/testify inconsitently and
sometimes mixed in the same tests. This can cause very strange issue,
e.g when using things like mock.MatchedBy().

This moves all our code to stretchr/testify, which seems to be far
more active and maintained then test-go/testify.

vendor/github.com/test-go/testify/require/forward_requirements.go

@@ -1,16 +0,0 @@
-package require
-
-// Assertions provides assertion methods around the
-// TestingT interface.
-type Assertions struct {
-	t TestingT
-}
-
-// New makes a new Assertions object for the specified TestingT.
-func New(t TestingT) *Assertions {
-	return &Assertions{
-		t: t,
-	}
-}
-
-//go:generate go run ../_codegen/main.go -output-package=require -template=require_forward.go.tmpl