Add dataframe creation test

tests/preprocessing_test.py

@@ -0,0 +1,9 @@
+from app.constants import files
+from app.preprocessing import create_dataframe
+from pandas import DataFrame
+
+
+def test_dataframe_creation():
+    for file in files.keys():
+        df = create_dataframe(file)
+        assert isinstance(df, DataFrame)