(core) Make regression tests by recording pycalls when opening fixture docs

Summary: Records all sandbox pycall arguments and results, saves them to JSON when the environment variable UPDATE_REGRESSION_DATA is set, otherwise checks that they match the saved JSON.

Test Plan: This is tests

Reviewers: dsagal, paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2851

sandbox/grist/objtypes.py

@@ -172,6 +172,8 @@ def encode_object(value):
       return str(value)
     elif isinstance(value, records.Record):
       return ['R', value._table.table_id, value._row_id]
+    elif isinstance(value, RecordStub):
+      return ['R', value.table_id, value.row_id]
     elif isinstance(value, datetime):
       return ['D', moment.dt_to_ts(value), value.tzinfo.zone.name if value.tzinfo else 'UTC']
     elif isinstance(value, date):