Try to fix travis issue for unit tests

Author: fdieulle

tests/__init__.py

@@ -0,0 +1,9 @@
+import os
+import clr
+import sys
+
+lib_folder = os.path.join(os.path.dirname(__file__), 'libs')
+if lib_folder not in sys.path:
+    sys.path.append(lib_folder)
+
+clr.AddReference('LibForTests')

tests/test_numpy_codecs.py

@@ -1,15 +1,8 @@
 import pandasnet
-import os
 import numpy as np
 import pytest
-import clr
 
-
-lib_file = os.path.join(os.path.dirname(__file__), 'libs', 'LibForTests.dll')
-if os.path.exists(lib_file):
-    clr.AddReference(lib_file)
-
-    from LibForTests import NumpyNet as npnet
+from LibForTests import NumpyNet as npnet
 
 
 @pytest.mark.skip(reason="pythonnet do not allow override")

tests/test_pandas_codecs.py

@@ -1,18 +1,12 @@
 import pandasnet
-import os
 import pandas as pd
 import numpy as np
 from datetime import datetime
-import clr
 import pytest
 
-lib_file = os.path.join(os.path.dirname(__file__), 'libs', 'LibForTests.dll')
-if os.path.exists(lib_file):
-    clr.AddReference(lib_file)
+from LibForTests import PandasNet as pdnet
 
-    from LibForTests import NumpyNet as npnet
 
-@pytest.mark.skip(reason="travis issue")
 def test_basic_dataframe():
     x = pd.DataFrame({
         'A': [1, 2, 3],
@@ -25,7 +19,7 @@ def test_basic_dataframe():
 
     __check(x, y)
 
-@pytest.mark.skip(reason="travis issue")
+
 def test_datetime64_into_dataframe():
     x = pd.DataFrame({
         'Utc': [