test: add unit tests for SubscribeMessageHandler

[cameri]

Description

1. Add unit tests to SubscribeMessageHandler

Related Issue

Part of #3

Motivation and Context

• Increase unit test coverage to above 80%

How Has This Been Tested?

1. Unit tests

Screenshots (if appropriate):

Types of changes

• Non-functional change (docs, style, minor refactor)
• 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.
• I have read the CONTRIBUTING document.
• I have added tests to cover my code changes.
• All new and existing tests passed.

[coveralls]

Pull Request Test Coverage Report for Build 3309002478

• 6 of 7 (85.71%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+5.9%) to 70.588%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/adapters/web-socket-adapter.ts	0	1	0.0%

Totals
Change from base Build 3301265399:	5.9%
Covered Lines:	466
Relevant Lines:	685

---

💛 - Coveralls