Fixes #39510 - Replace enzyme test in SkeletonLoader with RTL

[kfamilonidis]

Fixes #39510: Replace enzyme test in SkeletonLoader with RTL

[Lukshio]

Summary

Clean migration for SkeletonLoader. The four fixture cases become four explicit RTL tests, and coverage actually improves (adds an assertion that the empty state is not shown when children are present).

Critical (must fix before merge)

• None

Suggestions (should fix)

• Commit message uses Fixes #39510: ... - Foreman's documented convention (and what's used on develop, e.g. Fixes #38653 - Fix settings show endpoint) is Fixes #XXXXX - description with a hyphen, not a colon. Please reformat.

Nice to have (optional)

• Counting skeleton placeholders via the .react-loading-skeleton class couples the test to react-loading-skeleton's internal markup. Reasonable given the component has no accessible text while loading, just flagging for awareness if the library ever changes its DOM output.

Checklist gaps

• PR description doesn't call out the new empty-state-hidden-when-children assertion as an intentional coverage improvement over the old snapshot.

[kfamilonidis]

@Lukshio commit title fixed. Added a NOTE for the class assertion.