SERVER-12127 migrate js tests to jscore suite when not related to writes

Migrate js tests starting from j-z.
Include SERVER-12920 Update use_power_of_2.js

Signed-off-by: Matt Kangas <matt.kangas@mongodb.com>
SERVER-12127 Temporarily put back jstest in order not to lose test coverage.

Signed-off-by: Matt Kangas <matt.kangas@mongodb.com>