Inappropriate test for `AbstractEvaluator`

[styamamoto]

AbstractEvaluator.hp_result is treated as Path although it is dict or None.

Unrelated to this, but it is better to make hp_result a private variable, because the variable is not accessed directly from outside of AbstractEvaluator (if believing VSCode lookup).

https://github.com/aistairc/aiaccel/blob/05eb2538ba5748f9231245e18054e938f3bee6da/tests/unit/master_test/evaluator_test/test_abstract_evaluator.py#L46-L50

https://github.com/aistairc/aiaccel/blob/05eb2538ba5748f9231245e18054e938f3bee6da/tests/unit/master_test/evaluator_test/test_abstract_evaluator.py#L61-L66