reuse window object for faster tests

mocha.opts

@@ -1,2 +1 @@
---bail
 test/test.js

test/helpers.js

@@ -53,10 +53,13 @@ export function tryToReadFile(file) {
 	}
 }
 
-export function env() {
 const { window } = new JSDOM('<main></main>');
 global.document = window.document;
 
+export function env() {
+	window._svelteTransitionManager = null;
+	window.document.body.innerHTML = '<main></main>';
+
 	return window;
 }