Fix group by logic in grouped recency and stdev tests

[clausherther]

• expect_grouped_row_values_to_have_recent_data did not correctly apply grouping levels to grouped recency checks
  • Closes expect_grouped_row_values_to_have_recent_data is not correctly applying groupings #206
• expect_column_values_to_be_within_n_stdevs did not expect a list for the group_by, unlike other similar tests
  • Closes expect_column_values_to_be_within_n_stdevs: group_by should be a list #194