tests: reorganize CLI's `convert` tests

Create a `convert` directory under tests/cli, so we can have
multiple separate tests targeting specific features of the convert
command.

Move test_convert_args in there, this is the file containing the things
that are easy to test, comparing the output of --run-args with an
expected output.

Move the auto source discovery test in there too, as it's a feature of
the convert command.  Rename it to "auto source discovery grouping",
since that is mostly what it exercises, and we'll want to add tests for
other areas of auto source discovery.

Change-Id: I20e5085d6b41eec322b777f45be1fee87894fe91
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1808
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>