tests: improve flt.utils.trimmer/test_trimming to test streams without packets

Augment the test to make it test streams without packet support.  A new
parameter 'with-packet-msgs' must be passed to the test source component to
control whether it will emit packet messages.

There are now two configuration axis in this test (with and without
stream message clock snapshots, with and without packet messages), which
gives 4 configurations.  I think it's still manageable to have them all
written explicitly.  However, if we are to add a third configuration
axis, I think we'll need to refactor the test to avoid having all the
expected outputs written explicitly, as it will become too big to be
manageable.

Change-Id: I0b488aa3f1506e9d43f320c1643a65db5317d63c
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3106
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>