assert: make YAML dependency pluggable via build tags

Make the YAML dependency required for {assert,require}.YAMLEq{,f} pluggable.

The implementation can be selected using build tags:
- testify_yaml_default (default): gopkg.in/yaml.v3 is used, like before
- testify_yaml_fail: YAML deserialization is not implemented and always
  fails. So assert.YAMLEq always fails. This is useful if the test suite
  package doesn't use assert.YAMLEq (very common case).
- testify_yaml_custom: the github.com/stretchr/testify/assert/yaml
  package exposes an Unmarshal variable of type func([]byte, any) error
  (same as gopkg.in/yaml.v3) that allows to plug any alternate
  implementation. For example github.com/goccy/go-yaml.Unmarshal.
  This allows to avoid the link constraints of the license of
  gopkg.in/yaml.v3 (see PR #1120).

Usage: go test -tags testify_yaml_fail

To install the alternate implementation with testify_yaml_custom:

	//go:build testify_yaml_custom

	package my_pkg_test

	import (
		goyaml "github.com/goccy/go-yaml"
		"github.com/stretchr/testify/assert/yaml"
	)

	func init() {
		yaml.Unmarshal = goyaml.Unmarshal
	}

assert/assertions.go

@@ -19,7 +19,9 @@ import (
 
 	"github.com/davecgh/go-spew/spew"
 	"github.com/pmezard/go-difflib/difflib"
-	"gopkg.in/yaml.v3"
+
+	// Wrapper around gopkg.in/yaml.v3
+	"github.com/stretchr/testify/assert/yaml"
 )
 
 //go:generate sh -c "cd ../_codegen && go build && cd - && ../_codegen/_codegen -output-package=assert -template=assertion_format.go.tmpl"

assert/yaml/yaml_custom.go

@@ -0,0 +1,25 @@
+//go:build testify_yaml_custom && !testify_yaml_fail && !testify_yaml_default
+// +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 [gopkg.in/yaml.v3].
+//
+// In your test package:
+//
+//		import assertYaml "github.com/stretchr/testify/assert/yaml"
+//
+//		func init() {
+//			assertYaml.Unmarshall = func (in []byte, out interface{}) error {
+//				// ...
+//	     			return nil
+//			}
+//		}
+package yaml
+
+var Unmarshal func(in []byte, out interface{}) error

assert/yaml/yaml_default.go

@@ -0,0 +1,37 @@
+//go:build !testify_yaml_fail && !testify_yaml_custom
+// +build !testify_yaml_fail,!testify_yaml_custom
+
+// Package yaml is just an indirection to handle YAML deserialization.
+//
+// This package is just an indirection that allows the builder to override the
+// indirection with an alternative implementation of this package that uses
+// another implemantation of YAML deserialization. This allows to not either not
+// use YAML deserialization at all, or to use another implementation than
+// [gopkg.in/yaml.v3] (for example for license compatibility reasons, see [PR #1120]).
+//
+// Alternative implementations are selected using build tags:
+//
+//   - testify_yaml_fail: [Unmarshal] always fails with an error
+//   - testify_yaml_custom: [Unmarshal] is a variable. Caller must initialize it
+//     before calling any of [github.com/stretchr/testify/assert.YAMLEq] or
+//     [github.com/stretchr/testify/assert.YAMLEqf].
+//
+// Usage:
+//
+//	go test -tags testify_yaml_fail
+//
+// You can check with "go list" which implementation is linked:
+//
+//	go list -f '{{.Imports}}' github.com/stretchr/testify/assert/yaml
+//	go list -tags testify_yaml_fail -f '{{.Imports}}' github.com/stretchr/testify/assert/yaml
+//	go list -tags testify_yaml_custom -f '{{.Imports}}' github.com/stretchr/testify/assert/yaml
+//
+// [PR #1120]: https://github.com/stretchr/testify/pull/1120
+package yaml
+
+import goyaml "gopkg.in/yaml.v3"
+
+// Unmarshal is just a wrapper of [gopkg.in/yaml.v3.Unmarshal].
+func Unmarshal(in []byte, out interface{}) error {
+	return goyaml.Unmarshal(in, out)
+}

assert/yaml/yaml_fail.go

@@ -0,0 +1,18 @@
+//go:build testify_yaml_fail && !testify_yaml_custom && !testify_yaml_default
+// +build testify_yaml_fail,!testify_yaml_custom,!testify_yaml_default
+
+// Package yaml is an implementation of YAML functions that always fail.
+//
+// This implementation can be used at build time to replace the default implementation
+// to avoid linking with [gopkg.in/yaml.v3]:
+//
+//	go test -tags testify_yaml_fail
+package yaml
+
+import "errors"
+
+var errNotImplemented = errors.New("YAML functions are not available (see https://pkg.go.dev/github.com/stretchr/testify/assert/yaml)")
+
+func Unmarshal([]byte, interface{}) error {
+	return errNotImplemented
+}