Change heartbeat timeout to seconds in repl_rt_heartbeat test

The heartbeat timeout enforcement was recently updated to be
specified in seconds to match the documentation for that option. The
repl_rt_heartbeat test has since been failing since it still specified
the timeout in milliseconds.  This change makes the test use seconds
for the heartbeat timeout gets the test passing again.
Merge branch 'bugfix/repl_rt_heartbeat_seconds_timeout'