tests: disable automake serial_tests

tests: disable automake serial_tests

Serial mode is the old one and offers much less options for
running the tests. Generally our tests seem to work fine
with the newer parallel mode. The only reason we stuck with
serial_tests seems to be that we didn't like that it doesn't
output the test output by default. We could fix that with a
custom test driver. But will put that into a separate commit.

Change-Id: Ic7265d89142637b0963a6847c6beb06d9163bbb1
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Message-Id: <20231214111635.237429-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg27812.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>