tests/lib/conds: change `CreateClockCls` type to `std::function`

tests/lib/conds: change `CreateClockCls` type to `std::function`

The `CreateClockCls` callable type is currently a raw function pointer.
An upcoming patch will want to pass lambdas with captures, so change
`CreateClockCls` to be `std::function` instead.  Update the
`CreateClockCls` parameters to be non-const where we want to move the
value, and const-ref otherwise.

Remove the `noClockClass` function, and use an empty `CreateClockCls` to
mean "do not create a clock class".  I couldn't find how to do the
equivalent of the `createClockCls == noClockClass` with an
`std::function`, but using an empty `std::function` is clear enough (we
could have done it with a nullptr before too).

Change-Id: Ie30e42b68b5ffad566458933869e78da8ca01614
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12781
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>