Add sample coverage tests

[huyuxin0429]

Description

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[coveralls]

Pull Request Test Coverage Report for Build 6236318115

• 5 of 6 (83.33%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 0.218%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/string_calculator.rb	5	6	83.33%

Totals
Change from base Build 6220048984:	0.2%
Covered Lines:	5
Relevant Lines:	2298

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💛 - Coveralls