bt2: Add `_Clock*Const` classes and adapt tests

Split Python Clock Class related 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.

Makes all `_ClockSnapshot` object Const

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I81e4643c80e6e94445ae0e4948279891416a38ca
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1996
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>