Write functional tests for the InnoDB monitor counters in information_schema.innodb_metrics

As a preparation for the code refactoring in
MDEV-15706 Remove information_schema.innodb_metrics, innodb_monitor_enable, innodb_monitor_disable
we should write functional tests to cover all existing InnoDB monitor counters.

See the comments in MDEV-16580, which removed one useless counter.

Because information_schema.innodb_metrics appeared in MariaDB Server 10.0 (and MySQL 5.6), I think that the tests should be implemented in 10.0 already. Between MySQL 5.6 and 5.7, the following have been removed:

• buffer_LRU_batch_pages
• buffer_LRU_batch_total_pages
• buffer_LRU_batches
• buffer_flush_batch_rescan
• log_pending_log_writes
• metadata_mem_pool_size

The following have been added in MySQL 5.7 and could be accordingly added in MariaDB 10.2:

• buffer_LRU_batch_evict_pages
• buffer_LRU_batch_evict_total_pages
• buffer_LRU_batch_flush_avg_pass
• buffer_LRU_batch_flush_avg_time_est
• buffer_LRU_batch_flush_avg_time_slot
• buffer_LRU_batch_flush_avg_time_thread
• buffer_LRU_batch_flush_pages
• buffer_LRU_batch_flush_total_pages
• buffer_LRU_batches_evict
• buffer_LRU_batches_flush
• buffer_LRU_get_free_loops
• buffer_LRU_get_free_waits
• buffer_flush_adaptive_avg_pass
• buffer_flush_adaptive_avg_time_est
• buffer_flush_adaptive_avg_time_slot
• buffer_flush_adaptive_avg_time_thread
• buffer_flush_avg_pass
• buffer_flush_avg_time
• buffer_flush_n_to_flush_by_age
• ddl_log_file_alter_table
• ddl_sort_file_alter_table
• innodb_dict_lru_count
• innodb_rwlock_sx_os_waits
• innodb_rwlock_sx_os_waits
• innodb_rwlock_sx_spin_rounds
• innodb_rwlock_sx_spin_rounds
• innodb_rwlock_sx_spin_waits
• innodb_rwlock_sx_spin_waits
• log_padded
• log_pending_log_flushes