Reduce number of unit test binaries

We currently have 406 unique unit test binaries in the RHEL6 builder: https://evergreen.mongodb.com/task/mongodb_mongo_master_enterprise_rhel_62_64_bit_display_unittests!_4c95c46e1ef6de25e96d410369108110ce47c0e1_19_06_18_15_40_30

Each of these binaries is statically linked, and many of them link the same subset of code repeatedly.

Collapsing to a smaller number of unit test binaries based on a test-per-directory scheme resulted in a 50% reduction in build time for the unit test task.

Building the unit tests is currently one of the long poles in the makespan, so reducing this could significantly speed patch build results for developers.