Tests: Add stress test for multi session per UID

For this test, 10 applications are used with 5 sessions set with per UID
buffers.

A helper script is also added which launch applications with given
parameters. A README file is also present to help developers/users use
the stress tests correctly.

Stress tests save the stderr/stdout of the session daemon in a log file
and core dump created using the name found in core_pattern (in /proc).

Signed-off-by: David Goulet <dgoulet@efficios.com>