Add rudimentary test for funding_rate fix

tests/utils/test_funding_rate_migration.py

@@ -0,0 +1,29 @@
+from shutil import copytree
+
+from freqtrade.util.migrations import migrate_funding_fee_timeframe
+
+
+def test_migrate_funding_rate_timeframe(default_conf_usdt, tmp_path, testdatadir):
+
+    copytree(testdatadir / 'futures', tmp_path / 'futures')
+    file_4h = tmp_path / 'futures' / 'XRP_USDT_USDT-4h-funding_rate.feather'
+    file_8h = tmp_path / 'futures' / 'XRP_USDT_USDT-8h-funding_rate.feather'
+    file_1h = tmp_path / 'futures' / 'XRP_USDT_USDT-1h-futures.feather'
+    file_8h.rename(file_4h)
+    assert file_1h.exists()
+    assert file_4h.exists()
+    assert not file_8h.exists()
+
+    default_conf_usdt['datadir'] = tmp_path
+
+    # Inactive on spot trading ...
+    migrate_funding_fee_timeframe(default_conf_usdt, None)
+
+    default_conf_usdt['trading_mode'] = 'futures'
+
+    migrate_funding_fee_timeframe(default_conf_usdt, None)
+
+    assert not file_4h.exists()
+    assert file_8h.exists()
+    # futures files is untouched.
+    assert file_1h.exists()