Bug #20010 mysql-test-run.pl: --record and --require conflict (test fails)

- When --record is passed to mysqltest the whole testcase should be executed
as it normally is while the output form the test is acumulating
in ds_res. When test has finished ds_res should simply be written to the
specified result file(if any) instead of comapring it against the result file.
Simplify handling of --require and also the cecking of result files by splitting
check_result function into one  function 'check_require' that is specialised in
checking require's and leave 'check_result' to do just that.
- "mysqltest --record" has been considered unsafe, but with this really simple
logic, it should be safe to use.