Pass WEBSOCKETS_TESTS_TIMEOUT_FACTOR to tox.

Previously, despite being declared in .github/workflows/tests.yml,
it had no effect because tox insulates test runs from the environment.

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist =
+env_list =
     py38
     py39
     py310
@@ -12,6 +12,7 @@ envlist =
 
 [testenv]
 commands = python -W error::DeprecationWarning -W error::PendingDeprecationWarning -m unittest {posargs}
+pass_env = WEBSOCKETS_*
 
 [testenv:coverage]
 commands =