Bug 1824708 - Call setsockopt before bind, r=gbrown

`setsockopt` should be called before `bind` to make sure the port can be quickly reused later.

Differential Revision: https://phabricator.services.mozilla.com/D181253

testing/mochitest/runtests.py

@@ -2230,8 +2230,8 @@ toolbar#nav-bar {
 
     def findFreePort(self, type):
         with closing(socket.socket(socket.AF_INET, type)) as s:
-            s.bind(("127.0.0.1", 0))
             s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+            s.bind(("127.0.0.1", 0))
             return s.getsockname()[1]
 
     def proxy(self, options):