Avoid global / duplicate component initialization in diff_file_spec.js

The following discussion from !18917 (merged) should be addressed:

• @winh started a discussion: (+1 comment)

  Rather than always creating two component instances here (vmReadable, vmUnreadable), I would do

  vm.destroy();
  vm = createComponentWithStore(...)

  in the test where we need vmUnreadable below.

  ...and then optionally create a follow-up to remove the global initialization of vm.