MDEV-11802 preparation: Clean up the purge tests.

Revert the MDEV-4396 tweak to innodb.innodb_bug14676111.
We must fix the root cause instead.

Allow gcol.innodb_virtual_purge to run on a non-debug build
(If wait_innodb_all_purged.inc is used in a non-debug test,
it will have no effect.)

Add the test innodb.index_merge_threshold from MySQL 5.7.

mysql-test/include/wait_innodb_all_purged.inc

@@ -9,8 +9,8 @@
 #    --source include/wait_innodb_all_purged.inc
 #
 --source include/have_innodb.inc
---source include/have_debug.inc
 
+if (`select version() like '%debug%'`) {
 --disable_query_log
 
 let $wait_counter_init= 300;
@@ -57,3 +57,4 @@ if (!$success)
 }
 
 --enable_query_log
+}