Convert all unit tests to mocha syntax

This is a direct translation of the tests in tests/unit from nodeunit
format to mocha format.

The are all passing when NODE_ENV=testing
Switch make test-unit to run mocha
Remove old nodeunit package and tests

These have been superseded by the mocha tests in ./test
Run jshint against mocha's test directory
Delete test databases in parallel

I can't see any good reason to delete the test databases in series,
hopefully this change will speed up running the tests and avoid
timeouts.