Increase code coverage

Change log:
* Increase code coverage for test_exchange.py
* Move Exchange Unit tests files tests/exchange/
* Move RPC Unit tests files tests/rpc/

freqtrade/tests/exchange/test_exchange_bittrex.py

@@ -0,0 +1,32 @@
+# pragma pylint: disable=missing-docstring,C0103
+
+import pytest
+from requests.exceptions import ContentDecodingError
+
+from freqtrade.exchange import Bittrex
+
+
+def test_validate_response_success():
+    response = {
+        'message': '',
+        'result': [],
+    }
+    Bittrex._validate_response(response)
+
+
+def test_validate_response_no_api_response():
+    response = {
+        'message': 'NO_API_RESPONSE',
+        'result': None,
+    }
+    with pytest.raises(ContentDecodingError, match=r'.*NO_API_RESPONSE.*'):
+        Bittrex._validate_response(response)
+
+
+def test_validate_response_min_trade_requirement_not_met():
+    response = {
+        'message': 'MIN_TRADE_REQUIREMENT_NOT_MET',
+        'result': None,
+    }
+    with pytest.raises(ContentDecodingError, match=r'.*MIN_TRADE_REQUIREMENT_NOT_MET.*'):
+        Bittrex._validate_response(response)