test: update test to ensure rel_mean is reasonable

Author: xmatthias

tests/data/test_btanalysis.py

@@ -306,6 +306,7 @@ def test_combined_dataframes_with_rel_mean(testdatadir):
     assert df.iloc[0]["count"] == 2
     assert df.iloc[-1]["count"] == 2
     assert len(df) < len(data["BTC/USDT"])
+    assert df["rel_mean"].between(-0.5, 0.5).all()
 
 
 def test_combine_dataframes_with_mean_no_data(testdatadir):