[BE]: Write an unit test for mongo

[gop50k]

What

Add the following missing test section

• Mongo

What not

Implementation policy/Detail

Reference

• infrastructure/mongo/container.go
• infrastructure/mongo/model.go
• infrastructure/mongo/schema.go
• infrastructure/mongo/mongodoc/model.go
• infrastructure/mongo/mongodoc/schema.go

File that needs to be tested