Problem:

The test case innodb.sys_tablestats places the table t1
in the background drop list.  So when the next test case
attempts to create a table t1, it fails.  We need a
mechanism to wait for table t1 to be dropped for sure
at the end of test innodb.sys_tablestats.

Solution:

Introduced a new debug only system variable
innodb_background_drop_list_empty which will enable us
to wait till the background drop list becomes empty.

rb#10376 approved by Satya.