Extract `appendView`, `destroyView` for testing

Defining `appendView` and `destroyView` in multiple tests is verbose, 
repetitious, and copy-paste-error prone.
I'm OK with it (makes sense what it is doing), but is somewhat confusing to read (because we aren't destroying a view).  Maybe we can make the destroy helper more general (as in not in `ember-view` package)?  Or just alias the import here as `import { destroyView as destroy } from ".....";`.  What do you think?