test(sample-02): added unit tests

[nathanArseneau]

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: Unit tests for sample 02

What is the current behavior?

Issue Number: #1539

What is the new behavior?

added unit tests in sample 02-gateways

Does this PR introduce a breaking change?

• Yes
• No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 00d5c6a5-4319-48ba-8f7b-b717bbcdde33

• 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 92.667%

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Totals
Change from base Build f8fbebfc-ae32-4a4b-ab20-2be9b8108b43:	0.0%
Covered Lines:	6508
Relevant Lines:	7023

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💛 - Coveralls

[kamilmysliwiec]

lgtm