let's always disconnect a user connection before dropping the said user.
MariaDB is traditionally very tolerant to active connections
of the dropped user, which isn't the case for most other databases.
Let's avoid unintentionally spreading incompatible behavior
and disconnect before drop.
Except in cases when the test specifically tests such a behavior.
when a definer for SP/view is wrong - it shold be ER_MALFORMED_DEFINER,
not ER_NO_SUCH_USER
when one uses current_role as a definer or grantee but there's no
current role - it should be ER_INVALID_ROLE not ER_MALFORMED_DEFINER
when a non-existent user is specified - it should be ER_NO_SUCH_USER,
which should say "The user does not exist", not "Definer does not exist"
clarify ER_CANT_CHANGE_TX_CHARACTERISTICS to say what cannot be changed
