Tests: Ensure Bubble uses it's superclass's valid private API

[gottadiveintopython]

Maintainer merge checklist

• Title is descriptive/clear for inclusion in release notes.
• Applied a Component: xxx label.
• Applied the api-deprecation or api-break label.
• Applied the release-highlight label to be highlighted in release notes.
• Added to the milestone version it was merged into.
• Unittests are included in PR.
• Properly documented, including versionadded, versionchanged as needed.

Problem

Bubble currently overwrites some of GridLayout's internal properties: _fills_row_first, _fills_from_left_to_right and _fills_from_top_to_bottom. If any of them are renamed/removed, Bubble's should be renamed/removed as well. The problem is that the unittests don't say anything about this. So if someone only modifies GridLayout's, and forgot to modify Bubble's, Bubble might silently get broken.

What this PR does

This PR lets the unittests check if those properties actually exist in Bubble's super classes.
(I feel like there is a better way to solve the problem, so I put "WIP" in the title.)

[gottadiveintopython]

It may be too late, but I guess you think issubclass(Bubble, GridLayout) is unnecessary because we'll never replace the superclass with another, as it breaks the api. If so, yes, you may be right.

Also, I'll fix the error on unittest soon. (test_animation.py)

[matham]

we'll never replace the superclass with another, as it breaks the api.

Exactly.

Also, I'll fix the error on unittest soon. (test_animation.py)

👍