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

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt