Clear session tracker state between test cases

This isn't _necessary_ to have the tests run, but not doing so causes
any outstanding sessions to fail delivery after the server is torn down,
which leads to a bunch of warnings

tests/integrations/test_wsgi.py

@@ -13,7 +13,6 @@ class SentinelError(RuntimeError):
 
 
 class TestWSGI(IntegrationTest):
-
     def setUp(self):
         super(TestWSGI, self).setUp()
         bugsnag.configure(

tests/utils.py

@@ -31,9 +31,17 @@ def setUp(self):
         self.server.sessions_received = []
 
     def tearDown(self):
-        bugsnag.legacy.default_client.uninstall_sys_hook()
-        client = bugsnag.Client()
-        client.configuration.api_key = 'some key'
+        previous_client = bugsnag.legacy.default_client
+        previous_client.uninstall_sys_hook()
+
+        if previous_client.session_tracker.delivery_thread is not None:
+            previous_client.session_tracker.delivery_thread.cancel()
+            previous_client.session_tracker.delivery_thread = None
+
+        previous_client.session_tracker.session_counts = {}
+
+        client = bugsnag.Client(api_key='some key')
+
         bugsnag.legacy.default_client = client
         bugsnag.legacy.configuration = client.configuration