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/require/doc.go

@@ -0,0 +1,31 @@
+// Package require implements the same assertions as the assert package but
+// stops test execution when a test fails.
+//
+// # Example Usage
+//
+// The following is a complete example using require in a standard test function:
+//
+//	import (
+//	  "testing"
+//	  "github.com/stretchr/testify/require"
+//	)
+//
+//	func TestSomething(t *testing.T) {
+//
+//	  var a string = "Hello"
+//	  var b string = "Hello"
+//
+//	  require.Equal(t, a, b, "The two words should be the same.")
+//
+//	}
+//
+// # Assertions
+//
+// The require package have same global functions as in the assert package,
+// but instead of returning a boolean result they call [testing.T.FailNow].
+// A consequence of this is that it must be called from the goroutine running
+// the test function, not from other goroutines created during the test.
+//
+// Every assertion function also takes an optional string message as the final argument,
+// allowing custom error messages to be appended to the message the assertion method outputs.
+package require

vendor/github.com/stretchr/testify/require/forward_requirements.go

@@ -0,0 +1,16 @@
+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 sh -c "cd ../_codegen && go build && cd - && ../_codegen/_codegen -output-package=require -template=require_forward.go.tmpl -include-format-funcs"

vendor/github.com/stretchr/testify/require/require.go.tmpl

@@ -0,0 +1,6 @@
+{{.CommentRequire}}
+func {{.DocInfo.Name}}(t TestingT, {{.Params}}) {
+	if h, ok := t.(tHelper); ok { h.Helper() }
+	if assert.{{.DocInfo.Name}}(t, {{.ForwardedParams}}) { return }
+	t.FailNow()
+}

vendor/github.com/stretchr/testify/require/require_forward.go.tmpl

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

vendor/github.com/stretchr/testify/require/requirements.go

@@ -0,0 +1,29 @@
+package require
+
+// TestingT is an interface wrapper around *testing.T
+type TestingT interface {
+	Errorf(format string, args ...interface{})
+	FailNow()
+}
+
+type tHelper = interface {
+	Helper()
+}
+
+// ComparisonAssertionFunc is a common function prototype when comparing two values.  Can be useful
+// for table driven tests.
+type ComparisonAssertionFunc = func(TestingT, interface{}, interface{}, ...interface{})
+
+// ValueAssertionFunc is a common function prototype when validating a single value.  Can be useful
+// for table driven tests.
+type ValueAssertionFunc = func(TestingT, interface{}, ...interface{})
+
+// BoolAssertionFunc is a common function prototype when validating a bool value.  Can be useful
+// for table driven tests.
+type BoolAssertionFunc = func(TestingT, bool, ...interface{})
+
+// ErrorAssertionFunc is a common function prototype when validating an error value.  Can be useful
+// for table driven tests.
+type ErrorAssertionFunc = func(TestingT, error, ...interface{})
+
+//go:generate sh -c "cd ../_codegen && go build && cd - && ../_codegen/_codegen -output-package=require -template=require.go.tmpl -include-format-funcs"