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/assertion_forward.go.tmpl

@@ -0,0 +1,5 @@
+{{.CommentWithoutT "a"}}
+func (a *Assertions) {{.DocInfo.Name}}({{.Params}}) bool {
+	if h, ok := a.t.(tHelper); ok { h.Helper() }
+	return {{.DocInfo.Name}}(a.t, {{.ForwardedParams}})
+}