testing: add Output method to TB

Updates #59928
Fixes #73937

Change-Id: Ibf7ec61758edccd245841c3acc9096563b44fcd2
Reviewed-on: https://go-review.googlesource.com/c/go/+/677875
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

api/go1.25.txt

@@ -101,6 +101,7 @@ pkg testing, method (*F) Output() io.Writer #59928
 pkg testing, method (*T) Attr(string, string) #43936
 pkg testing, method (*T) Output() io.Writer #59928
 pkg testing, type TB interface, Attr(string, string) #43936
+pkg testing, type TB interface, Output() io.Writer #59928
 pkg testing/fstest, method (MapFS) Lstat(string) (fs.FileInfo, error) #49580
 pkg testing/fstest, method (MapFS) ReadLink(string) (string, error) #49580
 pkg testing/synctest, func Test(*testing.T, func(*testing.T)) #67434

src/testing/testing.go

@@ -900,6 +900,7 @@ type TB interface {
 	Skipped() bool
 	TempDir() string
 	Context() context.Context
+	Output() io.Writer
 
 	// A private method to prevent users implementing the
 	// interface and so future additions to it will not