Adds the ability to ''locally'' run visual regression testing for wp-admin pages via `npm run test:visual`. Snapshots are stored on contributors' local machines.
Note:
Wiring to the CI is not included. Why? The challenges for the CI are storage of the artifacts and unreliability of testing these across different environments.
This commit is a first step towards visual regression testing. Running it locally provides a learning opportunity which could help to craft how to build it into the automated CI process.
Props isabel_brison, andraganescu, azaozz, danfarrow, desrosj, hellofromTonya, justinahinon, netweb, talldanwp.
Fixes#49606.
git-svn-id: https://develop.svn.wordpress.org/trunk@51989 602fd350-edb4-49c9-b593-d223f7449a82
