Ensure that pending requests doesn't resolve prematurely

[jonaswalden]

// code.js
fetch("/fast-resource");
fetch("/slow-resource");

// test.js
await Promise.all(browser.window.fetch._pendingRequests);

Just pushing and popping the stack makes response of the /fast-resource pop the pending response for /slow-resource which causes the Promise.all() to resolve prematurely.

Having promises resolve with the arguments passed to fetch() is nice for debugging.