test: add more test coverage

[fadi-george]

Description

One Line Summary

• adds more tests and improves coverage to 20%

Details

• updates inappmessage src file to return jsonRepresentation
  • also updates to closingMessage to optional
  • makes handleMethod public for testing
• adds dlcov and rps dev dependencies
  • rps for running the custom test script in the pubspec yaml
  • dlcov for comparing against the total coverage

Motivation

• want to improve testing coverage for all sdks

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
  • If it is hard to explain how any codes changes are related to each other then it most likely needs to be more than one PR
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
  • Simplify with less code, followed by splitting up code into well named functions and variables, followed by adding comments to the code.
• I have reviewed this PR myself, ensuring it meets each checklist item
  • WIP (Work In Progress) is ok, but explain what is still in progress and what you would like feedback on. Start the PR title with "WIP" to indicate this.

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This change is