chore(deps): vendor testify

Add github.com/stretchr/testify (test-only) as a direct dependency and
vendor it, matching the traefikoidc precedent: _test.go files are never
Yaegi-interpreted, so vendoring a test-only dependency does not affect the
plugin's runtime under Traefik or the catalog analyzer.

tools/yaegi-check/main.go: add vendor to the harness's repo-copy exclude
list, since vendor/'s nested module layout (go.yaml.in/yaml/v3 alongside
testify) has nothing to do with the plugin package Yaegi imports.

Verified: make yaegi-check stays green; go build/vet/test unaffected.

vendor/github.com/stretchr/testify/internal/spew/README.md

@@ -0,0 +1,12 @@
+go-spew
+=======
+
+[![ISC License](http://img.shields.io/badge/license-ISC-blue.svg)](http://copyfree.org)
+
+Go-spew implements a deep pretty printer for Go data structures to aid in
+debugging.  A comprehensive suite of tests with 100% test coverage is provided
+to ensure proper functionality.
+
+## License
+
+Go-spew is licensed under the [copyfree](http://copyfree.org) ISC License.