Don't use app_context context manager while accessing configuration.

If we just want to access configuration, it's important to not use push
and pop methods of the actual app context object since that would mean
the cleanup handlers will be called.

Fix issue #2900.

flask/testing.py

@@ -16,7 +16,7 @@ from contextlib import contextmanager
 from click.testing import CliRunner
 from flask.cli import ScriptInfo
 from werkzeug.test import Client, EnvironBuilder
-from flask import _request_ctx_stack
+from flask import _request_ctx_stack, _app_ctx_stack
 from flask.json import dumps as json_dumps
 from werkzeug.urls import url_parse
 
@@ -78,9 +78,15 @@ def make_test_environ_builder(
             "Client cannot provide both 'json' and 'data'."
         )
 
-        # push a context so flask.json can use app's json attributes
-        with app.app_context():
+        # push a context so flask.json can use app's json attributes.
+        # Note that it's important to not use the push and pop methods
+        # of the actual app context object since that would mean the
+        # cleanup handlers will be called.
+        _app_ctx_stack.push(app.app_context())
+        try:
             kwargs['data'] = json_dumps(kwargs.pop('json'))
+        finally:
+            _app_ctx_stack.pop()
 
         if 'content_type' not in kwargs:
             kwargs['content_type'] = 'application/json'