The test seems to rely on the existence of some key. Don't rely on this, set some custom key before running the test.

Also, test the negative case: first, make sure using assert_exists the "test_assert_exists" key has no value (by checking the return code, this is the negative case). Then, set the "test_assert_exists" case to some value (using arakoon_set). Then, validate using assert_exists the key does exist.