Tests: Add controls to run python tests with verbose output

Tests: Add controls to run python tests with verbose output

When running failing tests, it can be useful to get verbose output
immediately without trying to run an environment with a separate
sessiond and/or relayd.

Setting `LTTNG_TEST_VERBOSE_RELAYD` or `LTTNG_TEST_VERBOSE_SESSIOND`
environment variables will cause the corresponding application to be run
in it's most verbose configuration.

Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Change-Id: Ic2dd84a36f61837dfbca99d06d6a438ae884f782
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>