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/assert/yaml/yaml_custom.go

@@ -0,0 +1,24 @@
+//go:build testify_yaml_custom && !testify_yaml_fail && !testify_yaml_default
+
+// Package yaml is an implementation of YAML functions that calls a pluggable implementation.
+//
+// This implementation is selected with the testify_yaml_custom build tag.
+//
+//	go test -tags testify_yaml_custom
+//
+// This implementation can be used at build time to replace the default implementation
+// to avoid linking with [go.yaml.in/yaml/v3].
+//
+// In your test package:
+//
+//		import assertYaml "github.com/stretchr/testify/assert/yaml"
+//
+//		func init() {
+//			assertYaml.Unmarshal = func (in []byte, out interface{}) error {
+//				// ...
+//	     			return nil
+//			}
+//		}
+package yaml
+
+var Unmarshal func(in []byte, out interface{}) error