It doesn't return null. It is an empty sequence. See renaming and refactoring suggested below.

Do we really need to have the expectedStages variable?

All other tests are written like this:

AssertStages(
 stages,
 """{ $sort : { _id : 1 } }""",
 """{ $group : { _id : null, _last : { $last : "$$ROOT" } } }""",
 """{ $replaceRoot : { newRoot : "$_last" } }"""); 

We also leave a blank between the translation assertions and the result assertions because they are asserting different things.

This is really two tests in one:

1. That the translation works as we expected it to
2. That the resulting MQL works on the server and returns the expected result

I know that the general recommendation is that every test should test one thing only, but we have chosen to be flexible in this case because otherwise we'd have twice as many tests, and each pair of tests would have identical setup.

Use single quote for fields, and then there will be no need for extra double-quotes outside.

Use a single quote ' inside of the string, then it will be no need to have multiple quotes:
"{ $group : { _id : null, _last : { $last : '$$ROOT' } } }"