Add TargetClient.TargetQAModeCookieName

[seeplusplus]

Description

There are already convenient getters on TargetClient for getting other AT cookie names. This adds the "at_qa_mode" cookie name as a constant.

Related Issue

Closes #114.

Motivation and Context

Provide a good developer experience. Adds similar interface for accessing QA Mode Cookie name as already exists with other Adobe Target cookies.

How Has This Been Tested?

Added a unit test.

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:

• I have signed the Adobe Open Source CLA.
• 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 changes.
• All new and existing tests passed.

[coveralls]

coverage: 91.026% (-1.5%) from 92.502%
when pulling fee0186 on seeplusplus:main
into a6e4f77 on adobe:main.