implement pkg handler exists test cases (#1628)

* add drafts of exists test cases

Signed-off-by: Kosuke Morimoto <ksk@vdaas.org>

* Update pkg/agent/core/ngt/handler/grpc/handler_test.go

Co-authored-by: Kevin Diu <kevindiujp@gmail.com>

* add pkg agent ngt handler exists test cases

Signed-off-by: Kosuke Morimoto <ksk@vdaas.org>

* fix missspellings

Signed-off-by: Kosuke Morimoto <ksk@vdaas.org>

Co-authored-by: Kevin Diu <kevindiujp@gmail.com>