bt2: Add remaining trace-ir `*Const` classes and adapt tests

Split Python classes into Const and Non-Const classes to mimic the type
safety offered by the C api. Const classes offer a read-only view of the
data. Non-Const classes subclass their respective Const classes.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I82b10d48bc9183d9a6bdcc6006ef0a886f9def83
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1924
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>