Adapt to new calling convention of ert--print-backtrace

Fixes #42

ert-runner.el

@@ -298,9 +298,18 @@ nil, `ert-runner-test-path' will be used instead."
                (ert-runner-message "Test %S backtrace:\n\n"
                                    (ert-test-name test))
                (with-temp-buffer
-                 (ert--print-backtrace
-                  (ert-test-result-with-condition-backtrace
-                   result))
+                 (let ((backtrace
+                        (ert-test-result-with-condition-backtrace result)))
+                   ;; The signature of ‘ert--print-backtrace’ has changed in
+                   ;; Emacs 26: it now takes two arguments.  We try to call it
+                   ;; with the new calling convention first and fall back to
+                   ;; the old one in case of error.
+                   ;; FIXME: We shouldn’t be using internal functions from
+                   ;; ert.el in the first place.
+                   (condition-case nil
+                       (ert--print-backtrace backtrace nil)
+                     (wrong-number-of-arguments
+                      (ert--print-backtrace backtrace))))
                  (goto-char (point-min))
                  (while (not (eobp))
                    (let ((start (point))