Refactor the testing of Window#build_panes

The previous specs seemed incomplete (i.e. when an Array is passed check
that Window#panes is not emtpy, and when a String is passed check that
it is an instance of Pane)

For the following two cases:

* The panes value is an Array
* The panes value is a String

I introduced the following specs:

* Check that the correct amount of Pane objects are being created
* Check that those created objects are returned by Window#panes
Ensure Window#panes always returns an Array

This is because it is treated like an Array in Window#panes? and in
lib/tmuxinator/assets/template.erb

Without this, the following YAML would fail:

windows:
  - vim:
      pre: "some_command"

and so would this:

windows:
  - vim:
    panes: "vim"