test: percy visual testing (#1053)

* test: percy visual testing init

* Try PR checks with custom command

* Only run percy visual tests for PWA on merge for now

platform/viewer/cypress/plugins/index.js

@@ -7,10 +7,10 @@
 // You can read more here:
 // https://on.cypress.io/plugins-guide
 // ***********************************************************
+let percyHealthCheck = require('@percy/cypress/task');
 
 // This function is called when a project is opened or re-opened (e.g. due to
 // the project's config changing)
-
 module.exports = (on, config) => {
   // `on` is used to hook into various events Cypress emits
   // `config` is the resolved Cypress config
@@ -24,4 +24,6 @@ module.exports = (on, config) => {
       return args;
     }
   });
+
+  on('task', percyHealthCheck);
 };