Add test for unset aliases after deactivate

Alias `pydoc` was not properly removed from the virtual environment
using the `unset` command.  We add a test after the environment
deactivation to confirm this fact.
Fix redundant notification from `unalias`

`unalias` reports an error if a specified alias is not already set.
We don't care about being informed on such occassion, since we do
remove `pydoc` (or other) aliases on any case.