There were two problems here:
  1. misleading error message
  2. abusing KILL QUERY in the test case

1. The server reported "'DELETE FROM t1' failed: 1689: Wait on a lock was
aborted due to a pending exclusive lock", while the proper error message
should be "'DELETE FROM t1' failed: 1317: Query execution was interrupted".

The problem is that the server has two different flags for
signalling that a query is being killed: THD::killed and
mysys_var::abort. The test case triggers a race: sometimes
mysys_var::abort is set earlier than THD::killed. That leads
to the following situation:

  - thr_lock() checks mysys_var::abort and returns error status,
    since mysys_var::abort is set;

  - the caller (mysql_lock_tables()) gets an error from thr_lock(),
    but THD::killed is not set, so it decides that thr_lock() couldn't
    get a lock due to a pending exclusive lock.

This is a known issue with the server and it's not going to be fixed soon.

5.5 differs from 5.1 here as follows: when thr_lock() returns an error:
  - 5.1 continues trying thr_lock() until success;
  - 5.5 propagates the error

2. The test case uses KILL QUERY is a highly concurent environment.

The fix is to wait for the dying statement to rest in peace before
executing another DELETE FROM t1.