Fix: Tests should set the LTTNG_SESSION_CONFIG_XSD_PATH variable

Tests should set the LTTNG_SESSION_CONFIG_XSD_PATH environment
variable to use the in-tree session configuration schema.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>