[BE] add mongo tests

[yk-eukarya]

Implement unit tests in internal/infrastructure/mongo

Refer to reearth/reearth-backend#139 to learn about how to write tests for mongo.

• asset.go (WIP: test: add Mongo Asset's FindByID unit testing reearth-backend#139)
• auth_request.go
• dataset.go
• dataset_schema.go
• layer.go
• lock.go
• plugin.go
• project.go (WIP: in test/project_repo_layer branch)
• property.go
• property_schema.go
• scene.go
• scene_lock.go
• tag.go
• team.go
• user.go
• transaction.go (you can ignore)

[rot1024]

MongoDB is so versatile that it is very difficult to mock, and even if mocked, it is not very useful.

Therefore, I think it is appropriate to implement internal/infrastructure/mongo/*_test.go as a unit test that works only when the MongoDB URL is specified in an environment variable. However, we need to consider whether it is a good idea to change the execution of the unit test depending on the environment variable.

[yuuri111]

I started writing unittest of [auth_request.go].