chore: add a onMount test

src/__tests__/fixtures/onMount.svelte

@@ -0,0 +1,12 @@
+<div data-testid="target">{message}</div>
+
+<script>
+    import { onMount } from 'svelte';
+
+    let message = "not yet";
+
+    onMount( () => {
+        message = 'nailed it';
+    });
+
+</script>

src/__tests__/onMount.test.js

@@ -0,0 +1,18 @@
+import { expect, test, vi } from 'vitest'
+
+import { render } from '..'
+import Comp from './fixtures/onMount.svelte'
+
+vi.mock('svelte', async () => {
+  const actual = await vi.importActual('svelte')
+  return {
+    ...actual,
+    onMount: (await import('svelte/internal')).onMount,
+  }
+})
+
+test('pretty prints the container', () => {
+  const { getByTestId } = render(Comp)
+
+  expect(getByTestId('target').innerHTML).toEqual('nailed it')
+})