bug#28333: 26.0.50; Backtrace not printed when ERT test fails

[Gemini Lasswell]

ERT used to print a backtrace for every failed test, but recently that
stopped happening. I git bisected and found that the commit which
introduced the problem is ead545824e, "Improve ert backtrace
recording".

Here's the test I've been using to reproduce the problem:

(require 'ert)
(defun div0 (n)
  (/ n (- n n)))
(ert-deftest failing-test ()
  "Check if backtrace works"
  (should (div0 1)))

When running the test interactively, pressing b at the test in ERT's
results buffer shows an empty backtrace. And here is the output of
running "make -C test fail" with the code above saved in
test/lisp/fail.el:

make: Entering directory '/home/gemini/src/emacs/test'
make[1]: Entering directory '/home/gemini/src/emacs/test'
  GEN      lisp/fail.log
Running 1 tests (2017-09-02 11:46:08-0700)
Test failing-test backtrace:
Test failing-test condition:
    (arith-error)
   FAILED  1/1  failing-test

Ran 1 tests, 0 results as expected, 1 unexpected (2017-09-02 11:46:08-0700)

1 unexpected results:
   FAILED  failing-test

make[1]: *** [Makefile:149: lisp/fail.log] Error 1
make[1]: Leaving directory '/home/gemini/src/emacs/test'
make: *** [Makefile:198: lisp/fail] Error 2
make: Leaving directory '/home/gemini/src/emacs/test'


In GNU Emacs 26.0.50 (build 6, x86_64-pc-linux-gnu, GTK+ Version 3.22.8)
 of 2017-09-02 built on chinook
Repository revision: ead545824e511ab18d18b5223eab80e1f4fe3d64
Windowing system distributor 'The X.Org Foundation', version 11.0.11902000
Recent messages:
Guessed variable ’python-indent’ (2)
You should have a section marked ";;; Commentary:"

The first line should be of the form: ";;; package --- Summary"
You can run the command ‘eval-buffer’ with M-x ev-b RET
Ran 1 tests, 0 results were as expected, 1 unexpected
You can run the command ‘ert’ with M-x ert RET
Ran 1 tests, 0 results were as expected, 1 unexpected
mouse-2, RET: Reveal test result
mouse-2, RET: Expand/collapse test result

Configured using:
 'configure --prefix=/home/gemini/src/emacs/out --with-modules
 --with-x-toolkit=gtk3 --with-xft --config-cache'

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND DBUS GSETTINGS NOTIFY LIBSELINUX
GNUTLS LIBXML2 FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 MODULES

Important settings:
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Major mode: ERT-View

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  buffer-read-only: t
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  column-number-mode: t
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Load-path shadows:
None found.

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Memory information:
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