innodb.drop_table_background failed in buildbot with "Tablespace for table exists"

innodb.drop_table_background 'xtradb'    w3 [ fail ]

        Test ended at 2017-06-09 06:04:25

CURRENT_TEST: innodb.drop_table_background

mysqltest: At line 29: query 'CREATE TABLE t (a INT) ENGINE=InnoDB' failed: 1813: Tablespace for table '`test`.`t`' exists. Please DISCARD the tablespace before IMPORT.

The result from queries just before the failure was:

CREATE TABLE t(c0 SERIAL, c1 INT, c2 INT, c3 INT, c4 INT,

KEY(c1), KEY(c2), KEY(c2,c1),

KEY(c3), KEY(c3,c1), KEY(c3,c2), KEY(c3,c2,c1),

KEY(c4), KEY(c4,c1), KEY(c4,c2), KEY(c4,c2,c1),

KEY(c4,c3), KEY(c4,c3,c1), KEY(c4,c3,c2), KEY(c4,c3,c2,c1)) ENGINE=InnoDB;

SET DEBUG_DBUG='+d,row_drop_table_add_to_background';

DROP TABLE t;

CREATE TABLE t (a INT) ENGINE=InnoDB;