Let us mask the actual values of the defragmentation-related fields,
because they may vary. Also, remove the dependency on purge,
and instead delete records by a ROLLBACK of INSERT.
btr_defragment_save_defrag_stats_if_needed(): Do not save
defragmentation statistics for temporary tables.
They are exempt of defragmentation anyway
(ha_innobase::optimize() never invokes defragmentation for them),
and the user-visible names are not available inside InnoDB.
Furthermore, InnoDB assumes that temporary tables are never accessed
by other threads than the one that handles the session with which
the temporary table is associated with.
Furthermore, we simplify the test innodb.innodb_defrag_stats
and include a test case that demonstrates that defragmentation
statistics are no longer being saved for temporary tables.
