test_{field,value}.py: add comments

test_{field,value}.py: add comments

Those files contain code which is very similar. It should be a single
helper base class eventually, but this patch only adds comments to both
files to clarify the usage of some classes and methods.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I15d8d4e1d8d3444491b9f01deb6d2cdcc5d406d1
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1584
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>