master 8dcb19f 4/4: Add a unit test testing interaction between threads and processes.

[Eli Zaretskii <eliz@gnu.org>]

This additional test fails on MS-Windows:

  Test process-tests/multiple-threads-waiting backtrace:
    signal(ert-test-failed (((should (eql (process-exit-status process)
    ert-fail(((should (eql (process-exit-status process) 0)) :form (eql
    (if (unwind-protect (setq value-344 (apply fn-342 args-343)) (setq f
    (let (form-description-346) (if (unwind-protect (setq value-344 (app
    (let ((value-344 'ert-form-evaluation-aborted-345)) (let (form-descr
    (let* ((fn-342 #'eql) (args-343 (condition-case err (let ((signal-ho
    (while (and (consp --cl-var--) (consp --cl-var--)) (progn (setq proc
    (let ((--cl-var-- processes) (process nil) (--cl-var-- threads) (thr
    (let ((threads nil) (cat (executable-find "cat"))) (let ((value-335
    (unwind-protect (let ((threads nil) (cat (executable-find "cat"))) (
    (let ((processes nil)) (unwind-protect (let ((threads nil) (cat (exe
    (progn (let ((processes nil)) (unwind-protect (let ((threads nil) (c
    (unwind-protect (progn (let ((processes nil)) (unwind-protect (let (
    (let* ((-with-timeout-timer- (run-with-timer 60 nil #'(lambda nil (t
    (catch 'timeout (let* ((-with-timeout-timer- (run-with-timer 60 nil
    (let ((-with-timeout-value- (catch 'timeout (let* ((-with-timeout-ti
    (closure (t) nil (let* ((fn-330 #'fboundp) (args-331 (condition-case
    ert--run-test-internal(#s(ert--test-execution-info :test #s(ert-test
    ert-run-test(#s(ert-test :name process-tests/multiple-threads-waitin
    ert-run-or-rerun-test(#s(ert--stats :selector ... :tests ... :test-m
    ert-run-tests((not (tag :unstable)) #f(compiled-function (event-type
    ert-run-tests-batch((not (tag :unstable)))
    ert-run-tests-batch-and-exit((not (tag :unstable)))
    eval((ert-run-tests-batch-and-exit '(not (tag :unstable))) t)
    command-line-1(("-L" ";." "-l" "ert" "-l" "src/process-tests.el" "--
    command-line()
    normal-top-level()
  Test process-tests/multiple-threads-waiting condition:
      (ert-test-failed
       ((should
	 (eql
	  (process-exit-status process)
	  0))
	:form
	(eql 1 0)
	:value nil))
     FAILED  21/26  process-tests/multiple-threads-waiting (8.859375 sec)

I did verify that process-send-eof does cause 'cat' to exit, so the
problem is likely in some issue related to threads and what the test
assumes regarding what should happen here.