Add integration tests for let variables in various commands

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.