cmd/testwrapper: fix exit deflake (#9342)

Sometimes `go test` would exit and close its stdout before we started reading
it, and we would return that "file closed" error then forget to os.Exit(1).
Fixed to prefer the go test subprocess error and exit regardless of the type of
error.

Fixes #9334

Signed-off-by: Paul Scott <paul@tailscale.com>

cmd/testwrapper/testwrapper.go

@@ -89,11 +89,6 @@ func runTests(ctx context.Context, attempt int, pt *packageTests, otherArgs []st
 		log.Printf("error starting test: %v", err)
 		os.Exit(1)
 	}
-	cmdErr := make(chan error, 1)
-	go func() {
-		defer close(cmdErr)
-		cmdErr <- cmd.Wait()
-	}()
 
 	s := bufio.NewScanner(r)
 	resultMap := make(map[string]map[string]*testAttempt) // pkg -> test -> testAttempt
@@ -164,10 +159,13 @@ func runTests(ctx context.Context, attempt int, pt *packageTests, otherArgs []st
 			}
 		}
 	}
-	if err := s.Err(); err != nil {
+	if err := cmd.Wait(); err != nil {
 		return err
 	}
-	return <-cmdErr
+	if err := s.Err(); err != nil {
+		return fmt.Errorf("reading go test stdout: %w", err)
+	}
+	return nil
 }
 
 func main() {
@@ -303,6 +301,8 @@ func main() {
 						os.Exit(exit.ExitCode())
 					}
 				}
+				log.Printf("testwrapper: %s", err)
+				os.Exit(1)
 			}
 		}
 		if len(toRetry) == 0 {