Refactor the testing of Window#build_panes
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

guyhughes/tmuxinator,ethagnawl/tmuxinator,marwensaid/tmuxinator,pczarn/tmuxinator,podung/tmuxinator,ethagnawl/tmuxinator,benizi/tmuxinator,jlipps/tmuxinator,J3RN/tmuxinator,podung/tmuxinator,ethagnawl/tmuxinator,pczarn/tmuxinator,benizi/tmuxinator,marwensaid/tmuxinator,sjfloat/tmuxinator,podung/tmuxinator,danimad/tmuxinator,sideci-sample/sideci-sample-tmuxinator,jlipps/tmuxinator,jlipps/tmuxinator,marcinmagier/tmuxinator,andyw8/tmuxinator,klaxalk/tmux,danimad/tmuxinator,pczarn/tmuxinator,azat/tmuxinator,cinaeco/tmuxinator,aMoniker/tmuxinator,sideci-sample/sideci-sample-tmuxinator,marcinmagier/tmuxinator,sachin21/tmuxinator,adamstrickland/tmuxinator,sachin21/tmuxinator,guyhughes/tmuxinator,andyw8/tmuxinator,sachin21/tmuxinator,cinaeco/tmuxinator,aMoniker/tmuxinator,marwensaid/tmuxinator,magicalbanana/tmuxinator,marcinmagier/tmuxinator,magicalbanana/tmuxinator,adamstrickland/tmuxinator,jasonchaffee/tmuxinator,adamstrickland/tmuxinator,jasonchaffee/tmuxinator,danimad/tmuxinator,aMoniker/tmuxinator,azat/tmuxinator,jasonchaffee/tmuxinator,klaxalk/tmux,tmuxinator/tmuxinator,magicalbanana/tmuxinator,klaxalk/tmux,sjfloat/tmuxinator,J3RN/tmuxinator,sjfloat/tmuxinator,azat/tmuxinator,klaxalk/tmux,tmuxinator/tmuxinator,guyhughes/tmuxinator,cinaeco/tmuxinator,benizi/tmuxinator,tmuxinator/tmuxinator,klaxalk/tmux,J3RN/tmuxinator