Install function for testing

Following discussions about testing the install script and @ljharb's will to have many small pull requests, here's the first one :)

What it does:

* wraps the code of the `install.sh` that was actually performing the install into a function
* The function is executed at install but __not__ during tests
* First test for the install script (_is the function defined?_)


Here's the complete list of functions to be `unset`

`nvm_do_install`
`nvm_has`
`nvm_download`
`install_nvm_as_script`
`install_nvm_from_git`