bug#56025: 29.0.50; em-extpipe-test-2 times out on EMBA and Cygwin

[Ken Brown <kbrown@cornell.edu>]

In commit 231a1ba3, Lars disabled the extpipe tests on EMBA "because they 
apparently time out".  The EMBA log he cited in the commit message shows that 
only em-extpipe-test-2 times out.  And this same test (but no others) also times 
out on Cygwin:

$ make -C test em-extpipe-tests
[...]
Test em-extpipe-test-2 backtrace:
   signal(error ("timed out waiting for subprocess(es)"))
   error("timed out waiting for subprocess(es)")
   eshell-wait-for-subprocess()
   eshell-command-result-p("echo \"bar\" | rev *>/tmp/emacs-test-Rw5ILv
   (let ((input (replace-regexp-in-string "temp\\([^>]\\|\\'\\)" temp (
   (unwind-protect (let ((input (replace-regexp-in-string "temp\\([^>]\
   (let ((temp-buffer (generate-new-buffer " *temp*" t))) (unwind-prote
   (progn (let ((temp-buffer (generate-new-buffer " *temp*" t))) (unwin
   (unwind-protect (progn (let ((temp-buffer (generate-new-buffer " *te
   (let* ((coding-system-for-write nil) (temp-file (identity (make-temp
   (save-current-buffer (set-buffer eshell-buffer) (let* ((fn-38 #'exec
   (unwind-protect (save-current-buffer (set-buffer eshell-buffer) (let
   (let* ((process-environment (cons "HISTFILE" process-environment)) (
   (progn (let* ((process-environment (cons "HISTFILE" process-environm
   (unwind-protect (progn (let* ((process-environment (cons "HISTFILE"
   (let* ((coding-system-for-write nil) (temp-file (file-name-as-direct
   (save-current-buffer (let* ((coding-system-for-write nil) (temp-file
   (let ((input "echo \"bar\" | rev *>temp")) (save-current-buffer (let
   (progn (let ((value-24 (gensym "ert-form-evaluation-aborted-"))) (le
   (closure (t) nil (progn (let ((value-24 (gensym "ert-form-evaluation
   ert--run-test-internal(#s(ert--test-execution-info :test #s(ert-test
   ert-run-test(#s(ert-test :name em-extpipe-test-2 :documentation nil
   ert-run-or-rerun-test(#s(ert--stats :selector ... :tests ... :test-m
   ert-run-tests((not (or (tag :unstable) (tag :nativecomp))) #f(compil
   ert-run-tests-batch((not (or (tag :unstable) (tag :nativecomp))))
   ert-run-tests-batch-and-exit((not (or (tag :unstable) (tag :nativeco
   eval((ert-run-tests-batch-and-exit '(not (or (tag :unstable) (tag :n
   command-line-1(("-L" ":../../master/test" "-l" "ert" "-l" "lisp/eshe
   command-line()
   normal-top-level()
Test em-extpipe-test-2 condition:
     (error "timed out waiting for subprocess(es)")
    FAILED  10/17  em-extpipe-test-2 (5.237846 sec) at 
../../master/test/lisp/eshell/em-extpipe-tests.el:87

I don't see what's special about this test that would cause it to time out on 
some systems.  Sean, do you have any ideas?  If not, I suggest just skipping it 
on Cygwin.  And maybe we should skip just this extpipe test (rather than all of 
them) on EMBA.

Ken