WIP: Add clientside feature flag test docs

What does this MR do?

Docs on recommended way to stub feature flags on the clientside, within the context of Karma and Jest

To answer these questions from contributors: https://gitlab.slack.com/archives/C0GQHHPGW/p1553111228499600

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer