Tests: Add environment variables in tests to attach gdbserver

Tests: Add environment variables in tests to attach gdbserver

When debugging tests, the current infrastructure in both the python and
bash test harnesses require that the user start the sessiond or relayd
beforehand, and run the tests with environment variables to stop the
spawning of the respective programs.

To facilitate the process, new environment variables are added to allow
gdbserver to be spawned and attach to the relayd or sessiond. The user
may then connect with gdb, for example: `gdb -ex "target
localhost:1001"`.

Change-Id: Id4d1b446c7d6682c011ef27682198fb4a503f5f4
Signed-off-by: kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>