CPopper(FR): Add tests for `CPopper` component

[codebender828]

Is your feature request related to a problem? Please describe.
Currently the CPopper component doesn't have any tests written for it. At the time of writing that component, I also did not have a very solid background on writing Vue component tests. So I wrote some it.todo statements to cover my basic expectation of the CPopper component.

Describe the solution you'd like

• Write a sensible set of unit tests for the CPopper component. (Should cover modifiers options that require some attention)

Additional context

• Related to fix: make CPopper's modifiers to be correctly merged #424