Add Chai assertion for NotFound route

[matthew-white]

Filing an issue based on getodk/central-frontend#1798 (comment)

It's a common pattern in tests for us to assert that we're on the NotFound route (or that we're not on the NotFound route). Currently, we do that like so:

app.findComponent(NotFound).exists().should.be.true; // or false

However, we could make tests more readable if we shortened/simplified that assertion. I'm thinking we could add a custom Chai helper to do so. I quite like the second suggestion from the PR comment above, something like:

app.should.be.NotFoundPage();

[Sufiyan-MSA]

Hey, can u pls assign me.