Ensure currentSelectedReviewers is always an array

What does this MR do and why?

When in single selection mode, the collapsible listbox returns a single item not wrapped in an array. In the rest of this feature, we expect it to be wrapped in an array, even in the case of there only being a single reviewer.

As part of this fix, correct the unit tests to occasionally emit a single string, as is the case in the non-multiselect version.

References

#586421 (closed)

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How to set up and validate locally

IN A FOSS GDK

1. Open MR
2. Attempt to change assigned reviewer
3. See successful reviewer change.

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