Create only necessary backstop reference during related test.

[jeawhanlee]

Is your feature request related to a problem? Please describe.
Right now, when we need to do VR testing we create references from the SCENARIO_URLS in wp.config irrespective of the test being run.

Describe the solution you'd like
It would make sense to conditionally create reference based on the current test being run, that way test run faster, less flaky on a poor internet connection with puppeteer trying to visit tons of urls and create reference and also we actually consume resource for only what we need.

I suggest we create a json or array file to hold various scenario urls related to their tests with their key as the script name e.g test:llcssbg this way we can get this script at run time and load it in SCENARIO_URLS in wp.config.ts just in time for backstop to create the references.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[jeawhanlee]

The suggestion in the issue should suffice for the grooming from my POV