net/http: avoid making a request to a closed server in TestServerShutdown
As soon as the test server closes its listener, its port may be reused
for another test server. On some platforms port reuse takes a long
time, because they cycle through the available ports before reusing an
old one. However, other platforms reuse ports much more aggressively.
net/http shouldn't know or care which kind of platform it is on —
dialing wild connections is risky and can interfere with other tests
no matter what platform we do it on.
Instead of making the second request after the server has completely
shut down, we can start (and finish!) the entire request while we are
certain that the listener has been closed but the port is still open
serving an existing request. If everything synchronizes as we expect,
that should guarantee that the second request fails.