(core) Fix adding multiple fields when editing card layout

Summary:
Bug reported by user (https://gristlabs.getgrist.com/k1f3bMzUvitZ/User-Feedback#a1.s3.r408.c8)
Also make the test of editing RecordLayout work better (it was failing on Mac even on master)

Test Plan: Added a test case for adding multiple fields

Reviewers: alexmojaki

Reviewed By: alexmojaki

Differential Revision: https://phab.getgrist.com/D3219

app/client/components/RecordLayout.js

@@ -254,7 +254,9 @@ RecordLayout.prototype.saveLayoutSpec = async function(layoutSpec) {
   // with items which require new columns first.
   let callbacks = addedCallbacks.concat(hiddenCallbacks);
   let positions = addedPositions.concat(hiddenPositions);
-  let addActions = gutil.arrayRepeat(addColNum, addColAction);
+
+  // Use separate copies of addColAction, since sendTableActions modified each in-place.
+  let addActions = gutil.arrayRepeat(addColNum, 0).map(() => addColAction.slice());
 
   await docData.bundleActions('Updating record layout.', () => {
     return Promise.try(() => {