Make our own mock kernel methods async (ipython#1346)

Author: Carreau

tests/conftest.py

@@ -121,7 +121,7 @@ def __init__(self, *args, **kwargs):
         self.shell = MagicMock()
         super().__init__(*args, **kwargs)
 
-    def do_execute(
+    async def do_execute(
         self, code, silent, store_history=True, user_expressions=None, allow_stdin=False
     ):
         if not silent: