setting SERVER_NAME does not restrict routing for both subdomain_matching and host_matching

[davidism]

I wrote a bunch in #5553 about investigating this. Essentially, we need to pass "" to disable subdomain matching, not None. The two tests that had to change were accidentally working because setting SERVER_NAME was bypassing subdomain_matching = False, and the tests should have been setting it to True since they were working with subdomains.

Now SERVER_NAME only affects routing if subdomain_matching=True, in which case it correctly limits routing to subdomains under that name.

fixes #5553

[davidism]

While trying to write a test, I discovered that SERVER_NAME interfered with host_matching as well. If host_matching=True, server_name should not be passed to bind_to_environ, otherwise it's always used instead of the request's Host.

[davidism]

I tested a corresponding change in Werkzeug to move subdomain_matching there, and this test still passes with no changes to Flask, and with changes to rely on the modified Werkzeug instead.