paxtest: install helper applications/libs to /usr/lib/paxtest

RUNDIR installs the test applications and shared libraries into a
different folder then /usr/lib. This is desired as there are a lot of
test apps which would get installed into /usr/lib without
organization. Instead, set RUNDIR=/usr/lib/paxtest to install the test
apps and libraries in a sub folder. The genpaxtest script accounts for
this path and handles a LD_LIBRARY_PATH update as part of the paxtest
script's exeuction.

Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>