Don't use threads in this test

I think test failures would've been ignored if there were some.

Fixes #1401

tests/test_basic.py

@@ -1180,8 +1180,9 @@ def test_test_app_proper_environ():
     assert rv.data == b'Foo SubDomain'
 
 
-def test_exception_propagation():
-    def apprunner(configkey):
+@pytest.mark.parametrize('config_key',
+                         ['TESTING', 'PROPAGATE_EXCEPTIONS', 'DEBUG', None])
+def test_exception_propagation(config_key):
     app = flask.Flask(__name__)
     app.config['LOGGER_HANDLER_POLICY'] = 'never'
 
@@ -1189,26 +1190,22 @@ def test_exception_propagation():
     def index():
         1 // 0
     c = app.test_client()
+
     if config_key is not None:
         app.config[config_key] = True
-            try:
+        with pytest.raises(Exception):
             c.get('/')
-            except Exception:
-                pass
-            else:
-                assert False, 'expected exception'
+
     else:
         assert c.get('/').status_code == 500
 
-    # we have to run this test in an isolated thread because if the
-    # debug flag is set to true and an exception happens the context is
-    # not torn down.  This causes other tests that run after this fail
+    # If the debug flag is set to true and an exception happens the context
+    # is not torn down.  This causes other tests that run after this fail
     # when they expect no exception on the stack.
-    for config_key in 'TESTING', 'PROPAGATE_EXCEPTIONS', 'DEBUG', None:
-        t = Thread(target=apprunner, args=(config_key,))
-        t.start()
-        t.join()
-
+    while flask._request_ctx_stack.top is not None:
+        flask._request_ctx_stack.pop()
+    while flask._app_ctx_stack.top is not None:
+        flask._app_ctx_stack.pop()
 
 def test_max_content_length():
     app = flask.Flask(__name__)