tests/test-gendoc-el.t

run-tests: don't append rtendtracing logs if pipe is /dev/null

If HGCATAPULTSERVERPIPE is not set, we set it to /dev/null, so we need to make
sure the value is not /dev/null before appending logs.

This fixes some of test-run-tests.t failures.

Differential Revision: https://phab.mercurial-scm.org/D4401

#require docutils gettext

  $ $TESTDIR/check-gendoc el
  checking for parse errors