feat(fluxtest): add support for flux feature flags in tests

[nathanielc]

For Flux acceptance tests its now possible to enable feature flags on a per test case basis.

• [x] Well-formatted commit messages
• [x] Rebased/mergeable
• [x] Tests pass
• [x] http/swagger.yml updated (if modified Go structs or API)
• [x] Feature flagged (if modified API)
• [x] Documentation updated or issue created (provide link to issue/pr)
• [x] Signed CLA (if not already signed)