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_test.go

@@ -4,6 +4,7 @@
 package testenv
 
 import (
+	"reflect"
 	"testing"
 
 	"tailscale.com/tstest/deptest"
@@ -29,3 +30,24 @@ func TestInParallelTestFalse(t *testing.T) {
 		t.Fatal("InParallelTest should return false before t.Parallel has been called")
 	}
 }
+
+// TestMatchesTestingTB verifies that TB has every exported method of
+// testing.TB, with matching signatures. It can't be a compile-time
+// assertion because testing.TB has an unexported method.
+func TestMatchesTestingTB(t *testing.T) {
+	want := reflect.TypeFor[testing.TB]()
+	got := reflect.TypeFor[TB]()
+	for m := range want.Methods() {
+		if m.PkgPath != "" {
+			continue // unexported
+		}
+		gm, ok := got.MethodByName(m.Name)
+		if !ok {
+			t.Errorf("TB lacks method %s%v", m.Name, m.Type)
+			continue
+		}
+		if gm.Type != m.Type {
+			t.Errorf("TB method %s has type %v; want %v", m.Name, gm.Type, m.Type)
+		}
+	}
+}