test: use con.list_tables for a more robust test

ibis/backends/tests/test_client.py

@@ -1727,7 +1727,6 @@ def test_insert_into_table_missing_columns(con, temp_table):
     assert result == expected_result
 
 
-@pytest.mark.notyet(["druid"], raises=AssertionError, reason="can't drop tables")
 @pytest.mark.notyet(
     ["clickhouse", "flink"],
     raises=AssertionError,
@@ -1742,30 +1741,24 @@ def test_insert_into_table_missing_columns(con, temp_table):
     ["bigquery"], raises=AssertionError, reason="test is flaky", strict=False
 )
 def test_memtable_cleanup(con):
-    memtables = con._memtables
-    memtable_count = len(memtables)
-
     name = ibis.util.gen_name("temp_memtable")
     t = ibis.memtable({"a": [1, 2, 3], "b": list("def")}, name=name)
-    op = t.op()
 
     # the table isn't registered until we actually execute, and since we
     # haven't yet executed anything, the table shouldn't be there
-    assert memtable_count == len(con._memtables)
+    assert name not in con.list_tables()
 
     # execute, which means the table is registered and should be visible in
     # con.list_tables()
     con.execute(t.select("a"))
-    assert len(con._memtables) == memtable_count + 1
+    assert name in con.list_tables()
 
     con.execute(t.select("b"))
-    assert len(con._memtables) == memtable_count + 1
+    assert name in con.list_tables()
 
     # remove all references to `t`, which means the `op` shouldn't be reachable
-    # and the table should thus be dropped and no longer visible in
-    # con.list_tables()
-    del t, op
-    assert len(con._memtables) == memtable_count
+    # and the table should thus be dropped and no longer visible
+    del t
     assert name not in con.list_tables()