add test

test/runtime/samples/store-resubscribe-observable/_config.js

@@ -0,0 +1,3 @@
+export default {
+	html: `42`,
+};

test/runtime/samples/store-resubscribe-observable/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	import { writable } from 'svelte/store';
+	function fake_observable(store) {
+		return { subscribe: cb => ({ unsubscribe: store.subscribe(cb) }) };
+	}
+
+	let foo = fake_observable(writable(0));
+	foo = fake_observable(writable(42));
+</script>
+
+{$foo}