tests: bump avocado version

tests: bump avocado version

Reverse debugging test uses gdb remote client of avocado framework.
This client was fixed since the currently used version 76.
Therefore this patch bumps the version to 81 and fixes command
line version compatibility issue.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgalyuk@ispras.ru>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <159903462803.28509.16851113546106095750.stgit@pasha-ThinkPad-X280>
Message-Id: <20200908202352.298506-2-philmd@redhat.com>
Message-Id: <20200909112742.25730-9-alex.bennee@linaro.org>