Ensure HTTP connections are closed in websocket tests

Differential Revision: https://phabricator.services.mozilla.com/D87494

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1653294
gecko-commit: 29be769939c2507fc75257e5bb09f067447eba47
gecko-integration-branch: autoland
gecko-reviewers: webdriver-reviewers, whimboo

tools/webdriver/webdriver/client.py

@@ -427,6 +427,8 @@ def start(self):
         if self.session_id is not None:
             return
 
+        self.transport.close()
+
         body = {"capabilities": {}}
 
         if self.requested_capabilities is not None:
@@ -452,6 +454,7 @@ def end(self):
             pass
         finally:
             self.session_id = None
+            self.transport.close()
 
     def send_command(self, method, url, body=None, timeout=None):
         """