make check: add warning when LTTng processes are running

This patch adds a warning when running LTTng/test processes are
detected at the beginning of make check. It offers a convenient
way to kill them and warns the user that some tests could fail
if the processes are kept alive.

The check can be disabled by setting
$LTTNG_TOOLS_TESTS_DISABLE_WARN_LTTNG_PROCESSES to 1

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>