Update assertion methods in testing guide

Adds documentation for these assertion methods to the testing guide:
- `assert_empty`
- `assert_not_empty`
- `assert_includes`
- `assert_not_includes`
- `assert_predicate`
- `assert_not_predicate`

I've also linked the minitest assertions documentation page for the full list of minitest assertions, as well as the minitest github repo.

/cc @senny