code: def test_vertical_concat_dfs():
      """Tests `vertical_concat_dfs`."""
      df0 = pd.DataFrame(
          {
              "ts": [0, 1, 2, 3, 4],
              "day": ["Mon", "Tue", "Wed", "Thu", "Fri"],
              "y": [10, 20, 30, 40, 50],
          }
      )
  
      df1 = pd.DataFrame(
          {
              "ts": [0, 1, 2, 3, 4],
              "day": ["Mon", "Tue", "Wed", "Thu", "Fri"],
              "y": [11, 21, 31, 41, 51],
          }
      )
  
      df = vertical_concat_dfs(
          df_list=[df0, df1],
          join_cols=["ts"],
          common_value_cols=["day"],
          different_value_cols=["y"],
      )
  
      expected_df = pd.DataFrame(
          {
              "ts": [0, 1, 2, 3, 4],
              "day": ["Mon", "Tue", "Wed", "Thu", "Fri"],
              "y0": [10, 20, 30, 40, 50],
              "y1": [11, 21, 31, 41, 51],
          }
      )
  
      assert pd.DataFrame.equals(df, expected_df)
  
imports: import itertools
  import numpy
  import pandas
  import pytest
  from testfixtures import LogCapture
  from greykite.common.constants import ANOMALY_COL
  from greykite.common.constants import END_TIME_COL
  from greykite.common.constants import LOGGER_NAME
  from greykite.common.constants import START_TIME_COL
  from greykite.common.constants import TIME_COL
  from greykite.common.constants import TimeFeaturesEnum
  from greykite.common.testing_utils import assert_equal
  from greykite.detection.detector.ad_utils import add_new_params_to_records
  from greykite.detection.detector.ad_utils import get_anomaly_df
  from greykite.detection.detector.ad_utils import get_anomaly_df_from_outliers
  from greykite.detection.detector.ad_utils import get_canonical_anomaly_df
  from greykite.detection.detector.ad_utils import get_timestamp_ceil
  from greykite.detection.detector.ad_utils import get_timestamp_floor
  from greykite.detection.detector.ad_utils import optimize_df_with_constraints
  from greykite.detection.detector.ad_utils import partial_return
  from greykite.detection.detector.ad_utils import validate_volatility_features
  from greykite.detection.detector.ad_utils import vertical_concat_dfs
label_desc: Use image node_id 3 for calling a global function with example usage: test_vertical_concat_dfs() without return types