HPCC-12663 Fix out of date assert in compaction unit tests

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6724 from ghalliday/issue12663b

HPCC-12663 Fix assert and optimize finding free page using _builtin_ffs

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>