Rspec tests

- `Assignment#assigned_groups`, `Assignment#unassigned_groups` and `Assignment#add_group` were migrated from rake tests to RSpec tests
- updated matchers that checked if two arrays were equal from `eq()` to `match_array()` so that order is not considered. This caused intermittent failing when the order of the retrieved records varied.
- improved readability and slightly optimized `Assignment#add_group` method. Both tests suites still pass with modifications.
Redundant `return` detected.