(core) Tests and bug fixes for bidirectional linking

Summary:
- Adding tests for bidirectional linking
- Fixing loop bug for bidirectional linking in custom widgets which use row filtering

Test Plan: New tests

Reviewers: JakubSerafin

Reviewed By: JakubSerafin

Differential Revision: https://phab.getgrist.com/D4070

app/plugin/grist-plugin-api.ts

@@ -77,11 +77,12 @@ export const allowSelectBy = viewApi.allowSelectBy;
  */
 export const setSelectedRows = viewApi.setSelectedRows;
 
-
+/**
+ * Sets the cursor position in a linked section.
+ */
 export const setCursorPos = viewApi.setCursorPos;
 
 
-
 /**
  * Fetches data backing the widget as for [[GristView.fetchSelectedTable]],
  * but decoding data by default, replacing e.g. ['D', timestamp] with