Add a mocked test case for `salt.modules.virtualenv_mod` python argument.

* Updated the `virtualenv_mod` mock test case to patch `salt.utils.which` and `sys.modules` at class level as opposed to every function.
Make sure we quote the prompt option to the `virtualenv_mod`.
Added test case for the prompt argument to `virtualenv_mod`.
Added mock test case for `clear`, `upgrade` and `symlinks` for the `virtualenv_mod`.