Ignore stderr when testing parallel autotest (#110351).

Ignore stderr when testing parallel autotest (#110351).

Non-release versions of bash (notably 5.1.0(1)-rc1, which was uploaded
to Debian unstable) print internal debugging messages like

    TRACE: pid 411364: bgp_delete: deleting 432074

to the test driver’s stderr while executing the parallel test driver.
This causes spurious failures in the test suite.  Chet Ramsey assures
me these are not a symptom of a bug in either bash or the driver code
(see https://lists.gnu.org/archive/html/bug-autoconf/2020-10/msg00047.html)
so have the test suite ignore them.

This fixes Savannah bug #110351.

* tests/autotest.at: Ignore stderr whenever running a micro-suite in
  parallel mode, to avoid spurious failures due to internal debugging
  messages that may be printed by bash.