Mark test-undo-region as unstable

Mark test-undo-region as unstable

It is repeatedly failing on Debian's arch:all autobuilders, though
seemingly nowhere else:

     passed  40/44  simple-transpose-subr (0.000396 sec)
  Test test-undo-region backtrace:
    signal(ert-test-failed (((should (= (length (delq nil (undo-make-sel
    ert-fail(((should (= (length (delq nil (undo-make-selective-list 1 9
    #f(compiled-function () #<bytecode 0x52f126616d2cdbd>)()
    ert--run-test-internal(#s(ert--test-execution-info :test #s(ert-test
    ert-run-test(#s(ert-test :name test-undo-region :documentation nil :
    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
    command-line-1(("-L" ":/<<PKGBUILDDIR>>/debian/build
    command-line()
    normal-top-level()
  Test test-undo-region condition:
      (ert-test-failed
       ((should
         (=
  	(length ...)
  	2))
        :form
        (= 3 2)
        :value nil))
     FAILED  41/44  test-undo-region (0.000185 sec)