t/rose-task-run/*-iso.t: fix mixed cylc 5/6 syntax

Also make `t/rose-suite-run/14-reload-null.t` more robust.
Merge pull request #1369 from matthewrmshin/fix-t-mixed-cylc-5-6

t/rose-task-run/*-iso.t: fix mixed cylc 5/6 syntax
rose host-select: improve performance

Don't SSH all hosts if ranking method is random and there is no
thresholds to consider. Shuffle host list, and return 1st available
host.