fix tests failing with server name warnings

After pallets/werkzeug#1577, mismatched configured and real server
names will show a warning in addition to raising 404. This caused
tests that did this deliberately to fail.

This patch removes the pytest fixture we were using to fail on
warnings, instead using the standard `-Werror` option. This speeds
up the tests by ~3x.

tests/test_helpers.py

@@ -443,7 +443,7 @@ class TestSendfile(object):
             assert rv.data == f.read()
         rv.close()
 
-    def test_send_file_xsendfile(self, app, req_ctx, catch_deprecation_warnings):
+    def test_send_file_xsendfile(self, app, req_ctx):
         app.use_x_sendfile = True
         rv = flask.send_file('static/index.html')
         assert rv.direct_passthrough