tests(sample-13): added unit tests

[Tony133]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: #1539

What is the new behavior?

Added unit tests in sample 13-mongo-typeorm

Does this PR introduce a breaking change?

• Yes
• No

Other information

This PR, added unit tests in sample 13-mongo-typeorm

[coveralls]

Pull Request Test Coverage Report for Build 86513fe9-e68e-4dff-a3bb-26d0f9bacf59

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.288%

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Totals
Change from base Build afe9516e-40fd-440f-8bbc-919eb37dd672:	0.0%
Covered Lines:	5530
Relevant Lines:	5865

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💛 - Coveralls

[jmcdo29]

Very basic tests, but LGTM

[kamilmysliwiec]

LGTM