tests: print executed CLI command line in bt_cli

tests: print executed CLI command line in bt_cli

When debugging a test, it is often useful to have the complete
CLI command line executed, so we can copy paste it and reproduce
the failure by hand.

This patch makes bt_cli echo the executed CLI command line to
stderr, such that it will show up when running one test by hand and in
the test logs when running "make check".

This allows simplifying bt_diff: we no longer have to pass the
CLI arguments to have them included in the error message, since
the full CLI command line will typically be printed just above.

Change-Id: I64f8c80f648d06319efe4e7544447183ba8069cc
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1839
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>