code: def test_timestamps_from_field(self):
      rowtime = Rowtime().timestamps_from_field("rtime")
  
      properties = rowtime.to_properties()
      expected = {
          "rowtime.timestamps.type": "from-field",
          "rowtime.timestamps.from": "rtime",
      }
      self.assertEqual(expected, properties)
  
imports: import collections
  import sys
  from pyflink.table.descriptors import Rowtime, Schema
  from pyflink.table.table_schema import TableSchema
  from pyflink.table.types import DataTypes
  from pyflink.testing.test_case_utils import PyFlinkTestCase
label_desc: Use image node_id 1 for calling the RowTimeDescriptorTests obj's underlying member method code with example usage: obj.test_timestamps_from_field() without return types