util/testenv: add ArtifactDir, Attr, Output methods to TB

The TB interface exists to mirror testing.TB without importing the
testing package, but it had fallen behind: Go 1.25 added Attr and
Output, and Go 1.26 added ArtifactDir. Add the missing methods and a
reflection-based test that TB has every exported method of testing.TB,
so future additions to testing.TB fail a test instead of silently
diverging. It can't be a compile-time assertion because testing.TB has
an unexported method.

Updates #16330
Updates #18682

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I9ba093afefdf3a6311ef4648bc1a13add9af453d

util/testenv/testenv.go

@@ -8,6 +8,7 @@ package testenv
 import (
 	"context"
 	"flag"
+	"io"
 
 	"tailscale.com/types/lazy"
 )
@@ -23,6 +24,8 @@ func InTest() bool {
 
 // TB is testing.TB, to avoid importing "testing" in non-test code.
 type TB interface {
+	ArtifactDir() string
+	Attr(key, value string)
 	Cleanup(func())
 	Error(args ...any)
 	Errorf(format string, args ...any)
@@ -35,6 +38,7 @@ type TB interface {
 	Log(args ...any)
 	Logf(format string, args ...any)
 	Name() string
+	Output() io.Writer
 	Setenv(key, value string)
 	Chdir(dir string)
 	Skip(args ...any)