Rickcsong/removes unnecessary tests

I remove a couple tests regarding changing the current selection by value.  It appears that it isn't used anywhere, but I'm not sure!  

There's still a couple of tests failing.  I'll try to fix them up later! :+1: 

Thank you so much for writing this! 
In Ember Select, the prompt is handled differently. It is treated as just another regular option.
Selectize can handle `placeholder` attribute. A useful test would be to assure that Selectize can recognize the placeholder somehow.