x/tools: make tests agnostic as to whether gotypesalias="" => 0 or 1

This is required temporarily as we flip the default.

Updates golang/go#65294

Change-Id: I552e40475cc48b949e2307af347ca98a428c55ea
Reviewed-on: https://go-review.googlesource.com/c/tools/+/578041
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

internal/testenv/exec.go

@@ -12,6 +12,7 @@ import (
 	"reflect"
 	"runtime"
 	"strconv"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -190,3 +191,22 @@ func Command(t testing.TB, name string, args ...string) *exec.Cmd {
 	t.Helper()
 	return CommandContext(t, context.Background(), name, args...)
 }
+
+// SkipMaterializedAliases skips the test if go/types would create
+// instances of types.Alias, which some tests do not yet handle
+// correctly.
+func SkipMaterializedAliases(t *testing.T, message string) {
+	if hasMaterializedAliases(Go1Point()) {
+		t.Skipf("%s", message)
+	}
+}
+
+func hasMaterializedAliases(minor int) bool {
+	if minor >= 23 && !strings.Contains(os.Getenv("GODEBUG"), "gotypesalias=0") {
+		return true // gotypesalias=1 became the default in go1.23
+	}
+	if minor == 22 && strings.Contains(os.Getenv("GODEBUG"), "gotypesalias=1") {
+		return true // gotypesalias=0 was the default in go1.22
+	}
+	return false // types.Alias didn't exist in go1.21
+}