bug#39067: shell-command-dont-erase-buffer strange behaviour

[Glenn Morris]

These tests fail for me on CentOS 8.1.

Test simple-tests-shell-command-39067 backtrace:
  signal(ert-test-failed (((should (string= expected (buffer-string)))
  ert-fail(((should (string= expected (buffer-string))) :form (string=
  #f(compiled-function () #<bytecode 0x6e2da1>)()
  ert--run-test-internal(#s(ert--test-execution-info :test #s(ert-test
  ert-run-test(#s(ert-test :name simple-tests-shell-command-39067 :doc
  ert-run-or-rerun-test(#s(ert--stats :selector ... :tests ... :test-m
  ert-run-tests((not (or (tag :expensive-test) (tag :unstable))) #f(co
  ert-run-tests-batch((not (or (tag :expensive-test) (tag :unstable)))
  ert-run-tests-batch-and-exit((not (or (tag :expensive-test) (tag :un
  eval((ert-run-tests-batch-and-exit '(not (or (tag :expensive-test) (
  command-line-1(("-L" ":." "-l" "ert" "-l" "lisp/simple-tests" "--eva
  command-line()
  normal-top-level()
Test simple-tests-shell-command-39067 condition:
    (ert-test-failed
     ((should
       (string= expected
                (buffer-string)))
      :form
      (string= "foo
" "Loading /usr/share/emacs/site-lisp/site-start.d/autoconf-init.el (source)...
Loading /usr/share/emacs/site-lisp/site-start.d/cmake-init.el (source)...
Loading /usr/share/emacs/site-lisp/site-start.d/desktop-entry-mode-init.el 
(source)...
Loading /usr/share/emacs/site-lisp/site-start.d/mercurial-site-start.el 
(source)...
Loading /usr/share/emacs/site-lisp/site-start.d/systemtap-init.el (source)...
../../../../../../usr/share/emacs/site-lisp/site-start.el: (lambda (dir) ...) 
quoted with ' rather than with #'
foo
")
      :value nil))
   FAILED  31/36  simple-tests-shell-command-39067 (1.691818 sec)
Test simple-tests-shell-command-dont-erase-buffer backtrace:
  signal(ert-test-failed (((should (= (point) (alist-get shell-command
  ert-fail(((should (= (point) (alist-get shell-command-dont-erase-buf
  #f(compiled-function () #<bytecode 0x6e2e15>)()
  ert--run-test-internal(#s(ert--test-execution-info :test #s(ert-test
  ert-run-test(#s(ert-test :name simple-tests-shell-command-dont-erase
  ert-run-or-rerun-test(#s(ert--stats :selector ... :tests ... :test-m
  ert-run-tests((not (or (tag :expensive-test) (tag :unstable))) #f(co
  ert-run-tests-batch((not (or (tag :expensive-test) (tag :unstable)))
  ert-run-tests-batch-and-exit((not (or (tag :expensive-test) (tag :un
  eval((ert-run-tests-batch-and-exit '(not (or (tag :expensive-test) (
  command-line-1(("-L" ":." "-l" "ert" "-l" "lisp/simple-tests" "--eva
  command-line()
  normal-top-level()
Test simple-tests-shell-command-dont-erase-buffer condition:
    (ert-test-failed
     ((should
       (=
        (point)
        (alist-get shell-command-dont-erase-buffer expected-point)))
      :form
      (= 517 5)
      :value nil))
   FAILED  32/36  simple-tests-shell-command-dont-erase-buffer (0.487757 sec)