Add tests for periodic jobs

[alanking]

There is currently no testing around periodic jobs (i.e. jobs which use the -i option of irods_sync start). Add some. Basic tests cases would include...

• Schedule sync, add something (and/or remove something!), wait for it to sync again
• Invalid interval values (non-numbers, negative values, too-large values, etc.)
• Jobs which take longer to complete than the specified interval to ensure that the same sync job is not getting scheduled simultaneously

I know in the relatively recent past we have started shying away from this feature, but it still exists, so it needs tests. If we don't want it to exist, we should deprecate and remove it.

[alanking]

Will probably be needed for #139