rsyslog: fix ptest

After rsyslog updated to 7.6.1, ptest fails to build & run since the
serial-tests config patch was dropped:

 - rsyslog-use-serial-tests-config-needed-by-ptest.patch

This patch involved serial test harness:

 + AM_INIT_AUTOMAKE([serial-tests])

Which is deprecated in favour of parallel test harness. Automake
generated a parallel test harness by default. It features automatic
collection of the test scripts output in .log files ...

More details please refer to:

 http://www.gnu.org/software/automake/manual/html_node/Parallel-Test-Harness.html



To enable the ptest with new changes, we should:

* add target 'buildtest-TESTS' for building test components;
* fix 'top_srcdir' and install required script 'test-driver';
* run testcases with target 'check-TESTS'.

Signed-off-by: Wenzong Fan <wenzong.fan@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>