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

@@ -1180,35 +1180,32 @@ def test_test_app_proper_environ():
     assert rv.data == b'Foo SubDomain'
 
 
-def test_exception_propagation():
-    def apprunner(configkey):
-        app = flask.Flask(__name__)
-        app.config['LOGGER_HANDLER_POLICY'] = 'never'
+@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'
 
-        @app.route('/')
-        def index():
-            1 // 0
-        c = app.test_client()
-        if config_key is not None:
-            app.config[config_key] = True
-            try:
-                c.get('/')
-            except Exception:
-                pass
-            else:
-                assert False, 'expected exception'
-        else:
-            assert c.get('/').status_code == 500
+    @app.route('/')
+    def index():
+        1 // 0
+    c = app.test_client()
 
-    # 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
-    # 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()
+    if config_key is not None:
+        app.config[config_key] = True
+        with pytest.raises(Exception):
+            c.get('/')
 
+    else:
+        assert c.get('/').status_code == 500
+
+    # 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.
+    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__)