Allow more control over tox selenium tests

Instead of unconditionally enabling selenium tests for the
py311-dj42-postgresql tox environment, enable them by default for that
environment but allow them to be disabled if the user's
DJANGO_SELENIUM_TESTS environment variable is empty.  This will allow
the user to run

    DJANGO_SELENIUM_TESTS= tox

to run the full tox test suite with selenium tests disabled.

docs/contributing.rst

@@ -85,6 +85,12 @@ or by setting the ``DJANGO_SELENIUM_TESTS`` environment variable::
     $ DJANGO_SELENIUM_TESTS=true make coverage
     $ DJANGO_SELENIUM_TESTS=true tox
 
+Note that by default, ``tox`` enables the Selenium tests for a single test
+environment.  To run the entire ``tox`` test suite with all Selenium tests
+disabled, run the following::
+
+    $ DJANGO_SELENIUM_TESTS= tox
+
 To test via ``tox`` against other databases, you'll need to create the user,
 database and assign the proper permissions. For PostgreSQL in a ``psql``
 shell (note this allows the debug_toolbar user the permission to create

tests/test_integration.py

@@ -547,7 +547,7 @@ def test_auth_login_view_without_redirect(self):
 
 @unittest.skipIf(webdriver is None, "selenium isn't installed")
 @unittest.skipUnless(
-    "DJANGO_SELENIUM_TESTS" in os.environ, "selenium tests not requested"
+    os.environ.get("DJANGO_SELENIUM_TESTS"), "selenium tests not requested"
 )
 @override_settings(DEBUG=True)
 class DebugToolbarLiveTestCase(StaticLiveServerTestCase):

tox.ini

@@ -36,7 +36,7 @@ passenv=
 setenv =
     PYTHONPATH = {toxinidir}
     PYTHONWARNINGS = d
-    py311-dj42-postgresql: DJANGO_SELENIUM_TESTS = true
+    py311-dj42-postgresql: DJANGO_SELENIUM_TESTS = {env:DJANGO_SELENIUM_TESTS:true}
     DB_NAME = {env:DB_NAME:debug_toolbar}
     DB_USER = {env:DB_USER:debug_toolbar}
     DB_HOST = {env:DB_HOST:localhost}