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/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"