Unit Tests for Actions in `packs` pack

[arm4b]

Following the ChatOps Aliases Tests, write Pack Unit Tests for packs Actions (#2982):

See: https://github.com/StackStorm/st2/tree/master/contrib/packs/actions

• packs.delete
• packs.download
• packs.get
• packs.get_config
• packs.load
• packs.search
• packs.setup_virtualenv
• packs.show
• packs.unload
• packs.update_virtualenv
• packs.virtualenv_prerun

Not going to do tests for these:
* [ ] packs.restart_component (it's just a local-shell-cmd)
* [ ] packs.uninstall - it's a chain calling delete/unload
* [ ] packs.install it's a chain calling download/virtualenv_prerun/setup_virtualenv/load