The db/pipeline/expression_*_test.cpp contain both tests for parsing expression and for ensuring that the evaluation returns the expected result. The latter should be part of a unit test living in the db/exec/expression folder

Files left out of SERVER-99443 are 

• expression_test.cpp
• expression_test_api_version_test.cpp
• expression_trigonometric_test.cpp
• expression_trim_test.cpp
• expression_walker_test.cpp