Tests: refuse to run test suite if lttng processes are present

The test suite often fails because of unclean environments where
stale LTTng processes are left running. Since the test suite
assumes that no LTTng process (daemons and test applications) are
running, it makes sense to force the user to kill all those
processes before running the test suite.

The warn_processes.sh script now prints an error and returns 1
to indicate an early failure to the test harness.

It is possible to circumvent this check by invoking the tests
manually or by removing the "exit 1" from the warn_processes.sh
script if there is a need to have persistent processes across
the execution of the test suite.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>