When I run mongo_embedded_test, it appears open the default dbpath (/data/db) with the default (or detected) storage engine. It then closes that storage engine before proceeding with the unit test suites (which all use temporary locations for their dbpaths).
This happens because the embedded instance initialize() calls initializeStorageEngine().

This means that if you have corrupt or problematic files in /data/db, your unit test can fail, or worse, silently change the files.