Enforce "serial-tests" behaviour for tests/Makefile

Enforce "serial-tests" behaviour for tests/Makefile

Our "make check" testsuite creates quite a bit of output which is
intended to help pinpointing the exact reason for failure - hidden by
default by automake 1.12 and up, which default to "parallel-tests" which
has no benefit for us.  So, just set the automake option to revert to
the old behaviour.

See also: https://www.gnu.org/software/automake/manual/html_node/Serial-Test-Harness.html

Trac #427

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Message-ID: <20150524193011.GK382@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9738
Acked-by: David Sommerseth <openvpn.list@topphemmelig.net>
(cherry picked from commit fc03ca9d13e35c40bdf1c3c676db2adf48c60223)