Test interesting edge cases for the 'let' variables. I think this should include:
- Test that the variable shadowing works correctly in aggregate for something like a conflicting name in a $let expression and in the 'let' parameter for a sub-pipeline for a $lookup.
- Test passing variables with reserved names (those that start with a capital letter), especially those that conflict with builtin variables like $$NOW.
- Test that variables can be used normally when querying a view.
- depends on
SERVER-46707 Support 'let' variables in aggregate command in sharded deployments
SERVER-46716 Support 'let' parameters for updates in sharded environments.