rose suite-run: test battery for pgrep check logic

Add test battery for the pgrep check (whether a suite is running or not)
logic. The test highlights a few problems which are fixed by this change:
* `pgrep` is now done on `localhost` as well as remote suite hosts.
* `pgrep` pattern is a bit more advanced. E.g. `rose suite-clean`
  would no longer fail on itself.
* The very similar `rose.suite_run.AlreadyRunningError` and
  `rose.suite_engine_proc.StillRunningError` are now combined.