lib: split "message-clock-classes-are-compatible" post-condition in multiple post...

Currently, `clock_classes_are_compatible()` returns a bool indicating if
the clock classes referenced in the current batch of messages are
compatible with the clock classes seen previously by the iterator.  If
there is any incompatibility, whatever the reason, the
"message-clock-classes-are-compatible" post-condition check is
triggered.

A subsequent patch will want to test each individual clock class
incompatibility case, which is not easy if they all produce a message
with the same condition id.

Change `clock_classes_are_compatible_one()` to directly do the
post-condition checks, and give a unique name to each check.

Rename to `assert_post_dev_clock_classes_are_compatible_one`, in line
with the existing `assert_post_dev_expected_packet()` & co.

Adjust and rename `clock_classes_are_compatible()` accordingly.

Change-Id: I38937422f731c0cfebad70f183be97144d43e121
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11954
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>