tests: rename some fields in test_value.py

tests: rename some fields in test_value.py

The very short names used in _TestIntegerValue and descendants were very
mysterious to me, it took me a while to figure out what each meant.
Rename them to slightly longer names for clarity.

Change-Id: I28dce959516d5bc91deadddb56e420e6b65d0277
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11410
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>