MDEV-4396: Fix sporadic failure of test innodb.innodb_bug14676111

The problem was that xtradb has innodb_purge_threads default 1 (plain
innodb defaults to 0).

The test sets a special debug variable and relies on it to force
purge to happen. But when using background purge threads, this does
not work, the debug code is not made to handle this, so occasionally
the test times out waiting for the purge to occur.

Fix by explicitly setting innodb_purgee_threads=0 for this test.

mysql-test/suite/innodb/t/innodb_bug14676111-master.opt

+--loose-innodb-purge-threads=0

mysql-test/suite/innodb/t/innodb_bug14676111.test

@@ -3,6 +3,11 @@
 -- source include/have_innodb.inc
 -- source include/have_debug.inc
 
+# Note that this test needs to be able to manipulate how/when purge is done
+# using @@innodb_limit_optimistic_insert_debug. This does not work with
+# background purge threads, so we disabled them in the -master.opt (they are
+# off by default in normal 5.5 innodb but on by default in xtradb)
+
 if (`select count(*)=0 from information_schema.global_variables where variable_name = 'INNODB_LIMIT_OPTIMISTIC_INSERT_DEBUG'`)
 {
   --skip Test requires InnoDB built with UNIV_DEBUG definition.