Fix test_dataframe when ran standalone (#546)

* Fix dataframe test when ran standalone

* Fix standalone tests in hyperopt and optimize tests

freqtrade/tests/test_dataframe.py

@@ -1,8 +1,10 @@
 # pragma pylint: disable=missing-docstring, C0103
 
 import pandas
+
 from freqtrade.optimize import load_data
 from freqtrade.analyze import Analyze
+from freqtrade.strategy.strategy import Strategy
 
 _pairs = ['BTC_ETH']
 
@@ -19,11 +21,13 @@ def load_dataframe_pair(pairs):
 
 
 def test_dataframe_load():
+    Strategy({'strategy': 'default_strategy'})
     dataframe = load_dataframe_pair(_pairs)
     assert isinstance(dataframe, pandas.core.frame.DataFrame)
 
 
 def test_dataframe_columns_exists():
+    Strategy({'strategy': 'default_strategy'})
     dataframe = load_dataframe_pair(_pairs)
     assert 'high' in dataframe.columns
     assert 'low' in dataframe.columns