fix the test of 'nvm deactivate'

It seems that the last condition of the test of `nvm deactivate` is wrong and the test itself has never passed. 

The problem is that the test supposes that after `nvm deactivate`, `nvm` itself fails. But the behaviour of `nvm deactivate` is only to remove paths for nvm-nodes and not to remove `nvm` itself. So the last condition should be `!(echo $PATH | grep nvm)`.

This PR fixes it.