After "lock table shared" explicit lock, "lock database exclusive" should fail.

The following statements will fail since tb1 and its database are locked in shared, and exclusive lock on database fails as expected.

use db1; 
lock table db1.tbl1 shared; 
lock database db1 exclusive;

While the following similar statements will pass just because the current database is different.

use default; 
lock table db1.tbl1 shared; 
lock database db1 exclusive;

Seems both case should fail.
Also check the test case lockneg_try_lock_db_in_use.q to add more reasonable failure cases.