Tests: bt2: move `_create_const_field()` to utils.py

Tests: bt2: move `_create_const_field()` to utils.py

So it can be used by other test cases. It will be used by the
`test_integer_range_set.py` file in a following commit.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ic64a60e9d1a7bca11292c824531949602d567a6f
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2117
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>