diff -u --label /tmp/test.el --label \#\<buffer\ /tmp/test.el\> /tmp/test.el /tmp/buffer-content-hY0EDf
--- /tmp/test.el
+++ #<buffer /tmp/test.el>
@@ -24,6 +24,7 @@
                :command '("wget" "-qO-" "https://en.wikipedia.org/wiki/Emacs")
                :connection-type 'pipe
                :sentinel #'test-sentinel)))
+    (set-process-thread proc (current-thread))
     (while (eq (process-status proc) 'run)
       (test-debug proc 'accept-output (accept-process-output proc 5)))
     (test-debug proc 'exit (process-status proc) (process-exit-status proc))))

Diff finished.  Sun Oct 14 16:04:48 2018