hack: block imports of vendored testify packages

While github.com/stretchr/testify is not used directly by any of the
repository code, it is a transitive dependency via Swarmkit and
therefore still easy to use without having to revendor. Add lint rules
to ban importing testify packages to make sure nobody does.

Signed-off-by: Cory Snider <csnider@mirantis.com>

.golangci.yml

@@ -45,6 +45,12 @@ linters-settings:
         deny:
           - pkg: io/ioutil
             desc: The io/ioutil package has been deprecated, see https://go.dev/doc/go1.16#ioutil
+          - pkg: "github.com/stretchr/testify/assert"
+            desc: Use "gotest.tools/v3/assert" instead
+          - pkg: "github.com/stretchr/testify/require"
+            desc: Use "gotest.tools/v3/assert" instead
+          - pkg: "github.com/stretchr/testify/suite"
+            desc: Do not use
   revive:
     rules:
       # FIXME make sure all packages have a description. Currently, there's many packages without.