fix: Default testId for Collapsible component (#686)

# Motivation

The `Collapsible` component renders `TestIdWrapper`. This last component
required a non-nullish `testId` (for obvious reasons). So, since this
seems to be the intention of the code, we default the `testId` to a
value.

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Co-authored-by: github-actions <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: David Dal Busco <[email protected]>

Author: 3 people

src/lib/components/Collapsible.svelte

@@ -9,7 +9,7 @@
   export let id: string | undefined = undefined;
   export let initiallyExpanded = false;
   export let maxContentHeight: number | undefined = undefined;
-  export let testId: string | undefined = undefined;
+  export let testId = "collapsible";
 
   export let iconSize: "small" | "medium" = "small";
   export let expandButton = true;