Tests: sink.text.pretty: Add unit tests for enum fields printing

Adds a Python component class who creates an event class with an enum
field that contains the enumeration described by the test caller. The
script iterator has a single event with a value also sent in parameter.

The tests themselves are run from a bash script, for signed/unsigned
values.

This commit also renames the existing `test_pretty` test file to
`test_pretty_python` to control all future Python tests for the
`sink.text.pretty` component class.

Change-Id: I3f631224dd3bdf21dbb2ef2d233c9f2dc8da43fa
Signed-off-by: Geneviève Bastien <gbastien@versatic.net>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3149
CI-Build: Francis Deslauriers <francis.deslauriers@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>