Assert which route names the dev server announces
The tests counted how many times a tab refetched, which the wrong
announcement name still produces, so five of the six passed with the fix
reverted. They now read the addedPage and removedPage frames off the HMR
socket and assert the names, and that nothing else is announced.
Renaming a route directory keeps the number of routes the same, which is
the only change the element-wise comparison in the webpack path decides,
so there's a test for that too.
The two comments say what the first-emission guards protect, which is a
window that closes before a client can connect, so no test reaches it.
