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 suite-clean --only=GLOBS

New `--only=GLOBS` option to restrict items to be cleaned.
Improve diagnostic.

rose test battery: use number of processors as number of jobs.
rose.suite_clean: improve diagnostics