Tests: Add high throughput with bandwidth limits test

This test is used to stress the new streaming feature with high
throughput in a bandwidth limited use case.

The bandwidth limitation is done via the tc (traffic control) kernel
utility. Root permissions are needed to set bandwidth limits.  Limits
are set on the loopback interface (lo).

The test cycle through bandwidth limits from 3200kbits to 50kbits.
There are currently two known limitations/issues:

- The tests fails when the lttng stop commands is issued right after the
  traced applications are done executing.

- Setting a bandwidth limit on the control port of the relayd will
  trigger a timeout in the sessiond.

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>