Add a test_unit target.
Add a test_unit target.

Especially useful for running the unit tests without a mongod and/or
postgres available.

jperichon/mosql,maheshm/mosql,NewAmsterdamLabs/mosql,iCHEF/mosql,mmasashi/mosql,stripe/mosql,ackinc/mosql,marty-stranger/mosql,ojosdegris/mosql