test(dropdown): add unit tests

tests/Dropdown/DropdownSlot.test.svelte

@@ -0,0 +1,25 @@
+<script lang="ts">
+  import { Dropdown } from "carbon-components-svelte";
+  import type { ComponentProps } from "svelte";
+
+  export let items: ComponentProps<Dropdown>["items"] = [
+    { id: "0", text: "Option 1" },
+    { id: "1", text: "Option 2" },
+    { id: "2", text: "Option 3" },
+  ];
+  export let selectedId: ComponentProps<Dropdown>["selectedId"] = "0";
+  export let id: ComponentProps<Dropdown>["id"] = "test-dropdown-slot";
+</script>
+
+<Dropdown
+  {items}
+  {selectedId}
+  {id}
+  titleText="Custom slot dropdown"
+  let:item
+  let:index
+>
+  <span data-testid="custom-item">
+    Item {index + 1}: {item.text}
+  </span>
+</Dropdown>