Bug#23921: random failure of user_limits.test

mysqltest disconnect/connect-combo could be so
quick that connect would hit the server before
it had processed the disconnect. Since that
resulted in one more concurrent connection than
we meant to have, global or per-user
max-user-connections could be exceeded.
This could lead to "random" failures in tests
that set those limits.


client/mysqltest.c:
  Retry max-connect-retries times if connect in
  connect_n_handle_errors() unexpectedly fails
  on connection-limit as this could be a race.
  Break out code that checks for expected --errors
  into its own function.
mysql-test/r/mysqltest.result:
  show that we throw a warning if test expects a SQL-state from
  a command (diff_files, ...) that clearly can't produce one.
mysql-test/t/disabled.def:
  re-enable user_limits
mysql-test/t/mysqltest.test:
  show that we throw a warning if test expects a SQL-state from
  a command (diff_files, ...) that clearly can't produce one.

mysql-test/t/mysqltest.test

@@ -91,6 +91,8 @@ select otto from (select 1 as otto) as t1;
 --error 1
 --exec echo "error S42S22; select otto from (select 1 as otto) as t1;" | $MYSQL_TEST  2>&1
 
+--error 1
+--exec echo "error S00000; remove_file $MYSQLTEST_VARDIR/tmp/test_nonexistent.tmp;" | $MYSQL_TEST  2>&1
 
 
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