innodb.innodb_buffer_pool_resize_temporary fails with (0/506)

The test reads innodb_buffer_pool_resize_status while the
SET GLOBAL innodb_buffer_pool_size thread is blocked at
buf_pool_shrink_before_wakeup and expects nothing withdrawn yet:

  Withdrawing blocks. (505/505).

But buf_pool_t::resize() sets n_blocks_to_withdraw and
first_to_withdraw and releases buf_pool.mutex before that sync
point. From then on other threads withdraw blocks: allocate() from
any page allocation, buf_LRU_block_free_non_file_page() on eviction,
and the page cleaner LRU batch which evicts pages from the withdrawn
area while to_withdraw is nonzero. So the first counter can be
anything down to 0:

  Withdrawing blocks. (0/506).

The fix masks the first counter in replace_regex, so the result
does not depend on how many blocks were already withdrawn.

This does not weaken the test: it still checks that the shrink is
reported while in progress. How many blocks are already withdrawn at
that moment depends only on timing.

mysql-test/suite/innodb/t/innodb_buffer_pool_resize_temporary.test

@@ -35,8 +35,9 @@ SET DEBUG_SYNC='buf_pool_shrink_before_wakeup SIGNAL blocked WAIT_FOR go';
 send SET GLOBAL innodb_buffer_pool_size=8388608;
 connection default;
 SET DEBUG_SYNC='now WAIT_FOR blocked';
-# adjust for 32-bit and SUX_LOCK_GENERIC
---replace_regex /(5..)\/\1/505\/505/
+# adjust for 32-bit and SUX_LOCK_GENERIC; other threads may already
+# have withdrawn some blocks before buf_pool_shrink_before_wakeup
+--replace_regex /\([0-9]+\/5[0-9][0-9]\)/(505\/505)/
 SHOW STATUS LIKE 'innodb_buffer_pool_resize_status';
 SET DEBUG_SYNC='now SIGNAL go';
 connection con1;