lib: merge `assert-pre.h` and `assert-post.h` into `assert-cond.h`

lib: merge `assert-pre.h` and `assert-post.h` into `assert-cond.h`

Merge both files into a single one to make it possible to reuse code.

Specific `BT_ASSERT_PRE*` and `BT_ASSERT_POST*` macros which were
identical are merged into a single one with the `BT_ASSERT_COND*` prefix
now.

No functional changes intended.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I019bcc73d5c644012238775c16b6498d07931a9a
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3428