Make test more explicit per suggestion by @znicholls

tests/test_core.py

@@ -132,12 +132,14 @@ def test_init_df_from_timeseries_unused_levels(test_df):
     # this test guards against regression for the bug
     # reported in https://github.com/IAMconsortium/pyam/issues/762
 
-    for _, df in test_df.timeseries().groupby(["model", "scenario"]):
+    for (model, scenario), data in test_df.timeseries().groupby(["model", "scenario"]):
         # we're only interested in the second model-scenario combination
-        pass
+        if model == "model_a" and scenario == "scen_b":
+            df = IamDataFrame(data)
 
-    # pandas does not remove unused levels (here: "Primary Energy|Coal") in a groupby
-    assert IamDataFrame(df).variable == ["Primary Energy"]
+    # pandas 2.0 does not remove unused levels (here: "Primary Energy|Coal") in groupby
+    # we check that unused levels are removed at initialization of the IamDataFrame
+    assert df.variable == ["Primary Energy"]
 
 
 def test_init_df_with_extra_col(test_pd_df):