Use new TS option to preserve imports in tests

These assertions were here to prevent imports (needed for svelte-inine-compile) from being removed.

src/lib/components/portal/portal.test.ts

@@ -13,9 +13,6 @@ beforeEach(() => {
 })
 
 it('should be possible to use a Portal', () => {
-  // Dummy assertion to trick TS compiler
-  expect(Portal).not.toBe(PortalGroup);
-
   expect(getPortalRoot()).toBe(null)
 
   render(svelte`