Add more expressiveness to E2E helpers expectJson and waitForJson

[nvie]

Extend E2E expressiveness by allowing to pass a predicate function to expectJson and waitForJson. Imagine:

await expectJson(page, "#connectionId", (value) => value > connId);
await waitForJson(page, "#connectionId", (value) => value > connId);

Maybe also nice to allow these predicates to be async functions, so we can more easily write those conditions as expressions that compare multiple selectors on the same page:

await waitForJson(page, "#someId", async (value) => value > (await getJson(page, "#otherId")));

Originates from #1195 (comment).