Problem: the test had not been updated after BUG#49978 was pushed
Fix: add 'set $rpl_only_running_threads= 1' to the end of the test
and update the result file.
Also, use include/assert.inc for an assertion (instead of relying
on result file comparison).
Also, move 'set @@global.slave_net_timeout' forwards, to get rid
of a warning.


mysql-test/suite/large_tests/r/rpl_slave_net_timeout.result:
  updated result file
mysql-test/suite/large_tests/t/rpl_slave_net_timeout.test:
   - Got rid of warning "The requested value for the heartbeat period
     exceeds the value of `slave_net_timeout' seconds. A sensible
     value for the period should be less than the timeout." by
     changing order of "set slave_net_timeout" and
     "change master to master_heartbeat_period".
   - replaced result file comparison by include/assert.inc
   - added "let $rpl_only_running_threads= 1"