Added findBy method and adjusted test message for existing findAllBy

Added findBy beneath find method.
Some messages for test code for findAllBy may be confusing after addition of findBy,
so adjusted to make clear to findAllBy is the kind of *All method.

Fixed a bug. From list to single.

findBy's result should be single, but used list.
Fixed a bug. From list to single.

findBy's result should be single, but used list.
Revert "Fixed a bug. From list to single."

This reverts commit f31f7dc570f969de1aeeb19ed58f270c5d1c963a.
Fixed a bug. Should use Option but used List.

Adjusted to a singple option result from reuse findAllBy code using list.
Added findBy method and adjusted test message for existing findAllBy

Added findBy beneath find method.
Some messages for test code for findAllBy may be confusing after addition of findBy,
so adjusted to make clear to findAllBy is the kind of *All method.

Fixed a bug. From list to single.

findBy's result should be single, but used list.

Fixed a bug. From list to single.

findBy's result should be single, but used list.
Modified test code.

Some test codes were not written or wrong.