Tests: Add the tracefile count test

This test validate the newly introduced tracefile count feature.  In
order to so, we enable a channel with the appropriate limits in
overwrite mode. The later constraint is to ensure that at least the last
few events are properly recorded and that we can validate that they are
present in the resulting trace (with the help of babeltrace and
babelstats).

We also validate that the tracefile count (per stream) in the trace
folder is conform to the one passed on the command line.

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