The additional patch. That 'loadxml.test' failure was actually about our testing system,
   not the code.
   Firstly we need a new mysqltest command, wich i called 'send_eval'. So the expression
   can be evaluated, then started in a parallel thread. We only have separane 'send' and
   'eval' commands at the moment.
   Then we need to add the waiting code after the 'KILL' to our test, so the thread will be killed
   before the test goes further. The present 'reap' command doesn't handle the killed threads
   well.
      
per-file comments:
  client/mysqltest.cc
Bug#42520      killing load .. infile Assertion failed: ! is_set(), file .\sql_error.cc, line 8
    The 'send_eval' command implemented.

  mysql-test/r/loadxml.result
Bug#42520      killing load .. infile Assertion failed: ! is_set(), file .\sql_error.cc, line 8
   test result updated.

  mysql-test/t/loadxml.test
Bug#42520      killing load .. infile Assertion failed: ! is_set(), file .\sql_error.cc, line 8
   test case added.