Fix: tests: error handling in high throughput limits test (v2)

Fix: tests: error handling in high throughput limits test (v2)

Each individual call to "tc" should be checked for error, else we
may fail to catch specific tc errors caused, for instance, by a
kernel configuration that only contains some of the required
class modules.

Also, invoke the utils.sh full_cleanup function from the script-specific
interrupt_cleanup trap handler rather than try to perform stopping
of relayd and sessiond within the script.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>