Test failure in thread-tests.el from emacs-26

[Juanma Barranquero]

This commit added a test in the release branch that fails because
there's no 'main-thread' variable in Emacs 26.

commit fdd1b3e02097966e7ffe8065f6e3d27f5f540d0a
Author: Eli Zaretskii <address@hidden>
Date:   2018-10-17 18:19:47 +0300

    Avoid assertion violation when comparing with main-thread

    * src/thread.c (unmark_main_thread): New function.
    * src/lisp.h (unmark_main_thread): Prototype it.
    * src/alloc.c (garbage_collect_1): Call it after sweeping.
    (Bug#33073)

    * test/src/thread-tests.el (threads-test-bug33073): New test.


Running the tests:

Test threads-test-bug33073 backtrace:
  signal(void-variable (main-thread))
  apply(signal (void-variable (main-thread)))
  [...etc...]
Test threads-test-bug33073 condition:
    (void-variable main-thread)
   FAILED  27/29  threads-test-bug33073