Mark esh-proc test as unstable

* test/lisp/eshell/esh-proc-tests.el
(esh-proc-test/pipeline-connection-type/last): Mark test as
unstable as it fails quite a lot.

test/lisp/eshell/esh-proc-tests.el

@@ -82,6 +82,8 @@ pipeline."
 
 (ert-deftest esh-proc-test/pipeline-connection-type/last ()
   "Test that only output streams are PTYs when a command ends a pipeline."
+  ;; Repeated unreproducible errors.
+  :tags '(:unstable)
   (skip-unless (executable-find "sh"))
   (eshell-command-result-equal
    (concat "echo | " esh-proc-test--detect-pty-cmd)